Polyacrylic acid (salt)-type water absorbent resin and method for producing of same

ABSTRACT

Disclosed is a method for producing a water absorbent resin, by which a surface-crosslinked water absorbent resin having excellent physical properties can be efficiently obtained at low cost, while assuring high productivity. When the production scale is increased to a continuous production on a huge scale (for example, 1 t/hr or more), the physical properties are improved and stabilized (for example, standard deviation of the physical properties is reduced) by a surface-crosslinking treatment, and the absorption against pressure (AAP) and liquid permeability (SFC) are further improved. Specifically disclosed is a method for producing a polyacrylic acid (salt)-type water absorbent resin, which comprises: a step of preparing an aqueous monomer solution using an acrylic acid (salt); a step of continuously polymerizing the aqueous monomer solution; shedding step of a hydrous gel-like crosslinked polymer during or after the polymerization; a step of drying the thus-obtained particulate hydrous gel-like crosslinked polymer; and a surface treatment step in which a surface-crosslinking agent is added to and reacted with the dried water absorbent resin powder. In the surface treatment step, the continuous mixing apparatus for the surface treatment agent and the continuous heating device are connected and periodic shielding is performed between the mixing apparatus and the heating device.

TECHNICAL FIELD

The present invention relates to a polyacrylic acid (salt)-type waterabsorbent resin and a method for producing the same. More particularly,the present invention relates to a polyacrylic acid (salt)-type waterabsorbent resin having high water absorption rate (CRC), high waterabsorption against pressure (AAP), and high liquid permeability (SFC)and containing little water extractables and a method for producing thesame.

BACKGROUND ART

A water absorbent resin (Super Absorbent Polymer; abbreviated as SAP)has been used in a wide range of uses for sanitary materials such aspaper diapers, sanitary napkins, incontinence products for adults, andthe like, and uses for water retention agent for soil, owing toproperties that the resin can absorb a large quantity of a water-basedliquid several times to several hundred times as much as the mass ofitself and has been manufactured and consumed in large quantities.

In general, a water absorbent resin is produced by polymerizing anaqueous solution containing a hydrophilic monomer and a crosslinkingagent to obtain a hydrous gel-like polymer, drying the gel polymer, andsurface-crosslinking the dried product. The physical properties such aswater absorption against pressure (AAP) and liquid permeability (GBP,SFC) of the above-mentioned water absorbent resin are improved bysurface-crosslinking step. The surface-crosslinking step is commonly astep of providing a highly crosslinked layer in the vicinity of thewater absorbent resin surface by causing reaction of the water absorbentresin with a surface-crosslinking agent or a polymerizable monomer.

Various kinds of surface-crosslinking agents reactive on a functionalgroup of a water absorbent resin (particularly, carboxyl group) areproposed as a surface-reforming method of such a water absorbent resinand examples known as the surface-crosslinking agents are oxazolinecompounds (Patent Document 1), vinyl ether compounds (Patent Document2), epoxy compounds (Patent Document 3), oxetane compounds (PatentDocument 4), polyhydric alcohol compounds (Patent Document 5), polyamidepolyamine-epihalo adducts (Patent Documents 6, 7), hydroxyacrylamidecompounds (Patent Document 8), oxazolidinone compounds (Patent Documents9, 10), bis- or poly-oxazoline compounds (Patent Document 11),2-oxotetrahydro-1,3-oxazolidine compounds (Patent Document 12), alkylenecarbonate compounds (Patent Document 13), and the like. A techniqueusing a specified surface-crosslinking agent (Patent Document 14) isalso known.

Techniques also known as the surface-reforming method other than themethod carried out by a surface-crosslinking agent are a technique ofsurface-crosslinking by polymerizing a monomer in the vicinity of thewater absorbent resin surface (Patent Document 15) and techniques ofradical crosslinking with persulfuric acid salts etc. (Patent Documents16, 17). Techniques of reforming water absorbent resins by heatingwithout using a surface-crosslinking agent (Patent Documents 18, 19),which is different from common surface-crosslinking treatment, are alsoknown.

A technique of using an additive in combination for mixing asurface-crosslinking agent is also proposed and examples known as theadditive are water-soluble cations such as aluminum salts and the like(Patent Documents 20, 21), alkali (Patent Document 22), organic acidsand inorganic acids (Patent Document 23), peroxides (Patent Document24), and surfactants (Patent Document 25).

Not only the chemical methods but also many surface treatment methodsusing apparatuses and reaction conditions have been proposed. Examplesknown as a method using an apparatus are techniques using a specifiedmixing apparatus as a mixing apparatus for a surface-crosslinking agent(Patent Documents 26 to 29) and techniques using a heating apparatus forcausing reaction of a water absorbent resin and a surface-crosslinkingagent (Patent Documents 30, 31) and the like.

There is also a technique for controlling an increase in heatingtemperature for causing reaction of a water absorbent resin and asurface-crosslinking agent (Patent Document 32) in improvement of thereaction condition aspect. In a heating step, techniques known are atechnique of carrying out surface-crosslinking twice (Patent Document33), a technique of controlling particle size by drying a waterabsorbent resin, thereafter carrying out a second heat drying step, andfurther carrying out surface-crosslinking (Patent Document 34), atechnique of defining oxygen partial pressure (Patent Document 35),techniques of defining the spraying conditions and dew points (PatentDocuments 37, 38), techniques of defining the mixing conditions oftreatment liquids (Patent Documents 39, 40), and a technique payingattention to a cooling step (Patent Document 41).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: U.S. Pat. No. 6,297,319

Patent Document 2: U.S. Pat. No. 6,372,852

Patent Document 3: U.S. Pat. No. 6,265,488

Patent Document 4: U.S. Pat. No. 6,809,158

Patent Document 5: U.S. Pat. No. 4,734,478

Patent Document 6: U.S. Pat. No. 4,755,562

Patent Document 7: U.S. Pat. No. 4,824,901

Patent Document 8: U.S. Pat. No. 6,239,230

Patent Document 9: U.S. Pat. No. 6,559,239

Patent Document 10: U.S. Pat. No. 6,503,979

Patent Document 11: U.S. Pat. No. 6,472,478

Patent Document 12: U.S. Pat. No. 6,657,015

Patent Document 13: U.S. Pat. No. 5,409,771

Patent Document 14: U.S. Pat. No. 5,422,405

Patent Document 15: US Patent Application Publication No. 2005/048221

Patent Document 16: U.S. Pat. No. 4,783,510

Patent Document 17: EP Patent No. 1824910

Patent Document 18: U.S. Pat. No. 5,206,205

Patent Document 19: EP Patent No. 0603292

Patent Document 20: U.S. Pat. No. 6,605,673

Patent Document 21: U.S. Pat. No. 6,620,899

Patent Document 22: U.S. Pat. No. 7,312,278

Patent Document 23: U.S. Pat. No. 5,610,208

Patent Document 24: US Patent Application Publication No. 2007/078231

Patent Document 25: US Patent Application Publication No. 2005/029352

Patent Document 26: U.S. Pat. No. 5,140,076

Patent Document 27: U.S. Pat. No. 6,071,976

Patent Document 28: US Patent Application Publication No. 2004/240316

Patent Document 29: WO No. 2007/065840 pamphlet

Patent Document 30: US Patent Application Publication No. 2007/149760

Patent Document 31:Japan Patent Application Publication No. 2004-352941

Patent Document 32: U.S. Pat. No. 6,514,615

Patent Document 33: U.S. Pat. No. 5,672,633

Patent Document 34: WO No. 2009/028568 pamphlet

Patent Document 35: US Patent Application Publication No. 2007/0293632

Patent Document 36: U.S. Pat. No. 6,720,389

Patent Document 37: U.S. Pat. No. 7183456

Patent Document 38: US Patent Application Publication No. 2007/161759

Patent Document 39: US Patent Application Publication No. 2006/057389

Patent Document 40: EP Patent No. 0534228

Patent Document 41: U.S. Pat. No. 7,378,453

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

It is difficult to satisfy demands from users for physical propertiessuch as water absorption against pressure and liquid permeability of awater absorbent resin only by a surface-crosslinking technique, althoughthere have been proposed many of surface-crosslinking agents (see PatentDocuments 1 to 13) and their combination use (see Patent Document 14),auxiliary agents for surface-crosslinking (see Patent Documents 20 to25), their mixing apparatuses (see Patent Documents 26 to 29) andheating apparatuses (Patent Documents 30, 31), and also various kinds ofconditions (see Patent Documents 32 to 41). Along with change of asurface-crosslinking agent and use of a new auxiliary agent, it may besometimes accompanied with an increase in cost, a decrease in safety,deterioration of other physical properties (e.g., coloration), and thelike in some cases. Although causing an effect to a certain extent in asmall scale or batch type production in an experimental laboratorylevel, the above-mentioned means may not sometimes show so mucheffective in an industrial scale (e.g., 1 t or more per unit hour) suchas large scale continuous production as compared with that in a smallscale.

The present invention has been completed in terms of the problems, andan object of the present invention is to provide a method for producinga water absorbent resin which is excellent in physical properties andsurface-crosslinked efficiently at a low cost while assuring highproductivity.

Solutions to the Problems

The inventors of the present invention have made investigations on asurface-crosslinking step for solving the above-mentioned problems andconsequently have solved the problems by carrying out heating treatmentwith a specified apparatus (preferably, with also a specified stirringpower index) in the surface treatment step after addition of asurface-crosslinking agent.

That is, the present invention provide a method for producing apolyacrylic acid (salt)-type water absorbent resin, comprising a step ofpreparing an aqueous monomer solution of an acrylic acid (salt), a stepof continuously polymerizing the aqueous monomer solution, a step offinely shredding a hydrous gel-like crosslinked polymer during or afterpolymerization, a step of drying the obtained particulate hydrousgel-like crosslinked polymer, and a surface treatment step of adding andreacting a surface treatment agent to and with the dried water absorbentresin powder, wherein in the surface treatment step, a continuous mixingapparatus for the surface treatment agent and a continuous heatingapparatus are connected and periodical shielding is carried out betweenthe mixing apparatus and the heating apparatus.

EFFECTS OF THE INVENTION

According to the present invention, in continuous production in a largeindustrial scale (particularly, treatment amount of 1 t/hr or more), thephysical properties (e.g., water absorption against pressure and liquidpermeability) can be improved after surface-crosslinking and thefluctuation of physical property (standard deviation) can be narrowed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing one example of theconfiguration of a heating apparatus or a cooling apparatus having abiaxial transverse type continuous stirring apparatus and being employedin the embodiment of the present invention.

FIG. 2 is a cross-sectional view showing one example of a stirring disk(equipped with scraping blades) of a biaxial heating apparatus orcooling apparatus.

FIG. 3 is a schematic view of the vertical cross section of a heatingapparatus and a cooling apparatus having a connected biaxial transversetype continuous stirring apparatus. Herein, the heating apparatus or thecooling apparatus is an apparatus having a similar shape with the sameinner area (inner volume) and can correspond to conventional techniques.

FIG. 4 is a cross-sectional view showing one example of a high speedrotation stirring type mixing apparatus usable for mixing asurface-crosslinking agent. Reference numeral 2 represents an innerwall, 6 represents a stirring shaft, and 7 (7 a, 7 b) representsstirring blades.

FIG. 5 is a cross-sectional view showing one example of a high speedrotation stirring type mixing apparatus usable for mixing asurface-crosslinking agent. Reference numeral 2 represents an innerwall, 6 represents a stirring shaft, and 7 (7 a, 7 b) representsstirring blades.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the polyacrylic acid (salt)-type water absorbent resin ofthe present invention and a method for producing the same will bedescribed in detail; however, the scope of the present invention is notrestricted to the following description, and those other than thefollowing examples can be properly modified and carried out in a rangewhere the gist of the present invention is not impaired. Specifically,the present invention is not limited to each of the followingembodiments, and various modifications can be made within a range shownby the claims and embodiments carried out by properly combining eachtechnical means disclosed with different embodiments are also includedwithin the technical scope of the present invention.

[1] Definition of Terms

(a) “Water Absorbent Resin”

The “water absorbent resin” means water-swelling and water-insoluble“polymer gelling agent” and includes those having the following physicalproperties. That is, those having, as the water-swelling property, awater absorption rate under no pressure (CRC) of 5 g/g or more. CRC ispreferably 10 to 100 g/g and more preferably 20 to 80 g/g. Because ofwater-insolubility, it is required that water extractables are in amountof 0 to 50 mass %. The water extractables are preferably in amount of 0to 30 mass %, more preferably in amount of 0 to 20 mass %, and evenpreferably in amount of 0 to 10 mass %.

The water absorbent resin is not limited to be embodiments of 100 mass %of a polymer and may contain other additives (described below) to theextent of retaining the above-mentioned characteristics. That is, even awater absorbent resin composition having the water absorbent resin andadditives is generally named as a water absorbent resin in the presentinvention. The content of the polyacrylic acid (salt)-type waterabsorbent resin is preferably 70 to 99.9 mass % relative to the entirewater absorbent resin, more preferably 80 to 99.7 mass %, and still morepreferably 90 to 99.5 mass %. The components other than the waterabsorbent resin are preferably water in terms of the water absorptionspeed and impact resistance of powder (particles) and may include, ifnecessary, additives described below.

(b) “Polyacrylic Acid (Salt)”

The “polyacrylic acid (salt)” means a polymer containing arbitrarily agraft component and, as a repeating unit, an acrylic acid (salt) as amain component. The acrylic acid (salt) as a monomer excluding acrosslinking agent is in an amount of essentially 50 to 100% by mole,preferably 70 to 100% by mole, more preferably 90 to 100% by mole, andstill more preferably substantially 100% by mole. The acrylic acid saltas the polymer essentially contains a polyacrylic acid salt andpreferably contains a monovalent salt, more preferably an alkali metalsalt or ammonium salt, still more preferably an alkali metal salt, andparticularly preferably sodium salt. The shape is not particularlylimited; however, the polyacrylic acid (salt) is preferably particles ora powder.

(c) “EDANA” and “ERT”

“EDANA” is an abbreviation of European Disposables and NonwovensAssociations. “ERT” is an abbreviation of measurement method (ERT/EDANARecommended Test Method) of a water absorbent resin on the basis ofEuropean Standards (almost Global Standards) as defined below. In thisspecification, unless otherwise specified, the physical properties of awater absorbent resin are measured based on ERT original text (KnownDocument: revised in 2002).

(c-1) CRC (ERT441.2-02)

The “CRC” is an abbreviation for Centrifuge Retention Capacity and meanswater absorption rate under no pressure (simply sometimes referred to as“water absorption rate”). Specifically, the CRC is the water absorptionrate (unit; g/g) after 0.200 g of a water absorbent resin in a nonwovenfabric bag is freely swollen in 0.9 mass % saline solution for 30minutes and dewatered by a centrifuge at 250 G.

(c-2) AAP (ERT442.2-02)

The “AAP” is an abbreviation for Absorption Against Pressure and meanswater absorption against pressure. Specifically, the APP is the waterabsorption rate (unit; g/g) after 0.900 g of a water absorbent resin isswollen in 0.9 mass % saline solution for 1 hour under 1.9 kPa load. Inthe present invention and examples, the measurement is carried out at4.8 kPa.

(c-3) “Extractables” (ERT 470.2-02)

“Extractables” means the amount of water soluble components (dissolveamount). Measurement is carried out by adding 1.000 g of the waterabsorbent resin to 200 ml of an 0.9 mass % aqueous saline solution,stirring the solution for 16 hours, and measuring the amount of adissolved polymer by pH titration (unit: mass %).

(c-4) Residual Monomers (ERT410.2-02)

The “residual monomers” means the amount of monomers remaining in awater absorbent resin. Specifically, the amount of monomers is a value(unit; ppm by mass) obtained by measuring, after 1.000 g of a waterabsorbent resin is charged to 200 cm³ of 0.9 mass % saline solution andthe resultant is stirred for 2 hours, the amount of monomers eluted inthe aqueous solution by using high-pressure liquid chromatography.

(c-5) PSD (ERT420.2-02)

The “PSD” is an abbreviation for Particle Size Distribution and meansthe particle size distribution measured by sieving classification. Themass average particle diameter and the particle diameter distributionwidth can be measured by the same method as in “(1) Average ParticleDiameter and Distribution of Particle Diameter” described in EuropeanPatent No. 0349240, p. 7, lines 25-43 and WO 2004/069915.

(c-6) Others

“pH” (ERT400.2-02): The “pH” means pH of a water absorbent resin.

“Moisture Content” (ERT 430.2-02) The moisture content means the watercontent of a water absorbent resin.

“Flow Rate” (ERT 450.2-02) The flow rate means the flow down speed of awater absorbent resin powder.

“Density” (ERT 460.2-02) The density means the bulk specific density ofa water absorbent resin.

(d) “Liquid Permeability”

The “liquid permeability” means the flow of a liquid flowing amongparticles of swollen gel under a load or no load. The “liquidpermeability” can be measured by SFC (Saline Flow Conductivity) or GBP(Gel Bed Permeability) as a representative measurement method.

The “SFC” is liquid permeability of 0.69 mass % physiological salinesolution in a water absorbent resin at a load of 0.3 psi. It is measuredaccording to an SFC testing method described in U.S. Pat. No. 5,669,894.

The “GBP” is liquid permeability of 0.69 mass % physiological salinesolution in a water absorbent resin under a load or free expansion. Itis measured according to a GBP testing method described in WO2005/016393 pamphlet.

(e) “Standard Deviation”

The “standard deviation” is a numeral value showing the degree ofdispersion of data and means a positive square root of the valuecalculated by totalizing the square value of the difference of the valueof data of n samples and their arithmetic average, that is, thedeviation, and dividing the total by n−1. It is used for understandingthe degree of fluctuation for the phenomenon with considerablefluctuation. In this specification, the standard deviation is employedfor digitalization of the fluctuation (deflection) for a desiredphysical value of interest.

$\begin{matrix}{\mspace{20mu} {{{{Data}\mspace{14mu} {of}\mspace{14mu} n\mspace{14mu} {samples}\mspace{14mu} x_{1}},x_{2},\ldots \mspace{14mu},x_{n}}\mspace{20mu} {{{Arithmetic}\mspace{14mu} {average}\mspace{14mu} X} = {\frac{1}{n}\text{?}\text{?}}}\mspace{20mu} {{{Standard}\mspace{14mu} {deviation}} = \sqrt{\frac{1}{n - 1}\text{?}\left( {{Xi} - X} \right)^{2}}}{\text{?}\text{indicates text missing or illegible when filed}}}} & \left\lbrack {{Numeral}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(f) Others

In this specification, “X to Y” showing a range means “X or more and Yor lower”. Additionally, the unit of mass “t (ton)” means “Metric ton”.Further, the measurement of physical properties of a water absorbentresin is carried out under the conditions of a temperature of 20 to 25°C. (sometimes simply referred to as “room temperature” or “normaltemperature”) and a relative humidity of 40 to 50%, unless otherwisestated.

[2] A Method for Producing Polyacrylic Acid (Salt)-Type Water AbsorbentResin

(1) Polymerization Step

(a) Monomer (Excluding a Crosslinking Agent)

A monomer of the present invention contains the above-mentioned acrylicacid or its salt as a main component and in terms of water absorptioncharacteristics and decrease of the residual monomers, the acid groupsof a polymer are preferable to be neutralized and the neutralizationratio is 10 to 100% by mole preferable, more preferably 30 to 95% bymole, still more preferably 50 to 90% by mole, and particularlypreferably 60 to 80% by mole, The neutralization may be carried out forthe polymer (hydrogel) after polymerization or for the monomer; however,in terms of productivity and improvement of AAP, neutralization of themonomer is preferable. Consequently, the monomer in the presentinvention includes a partially neutralized salt of the acrylic acid.

Further, in the present invention, a hydrophilic or hydrophobicunsaturated monomer may be used besides an acrylic acid (salt). Monomersusable may include methacrylic acid, maleic acid, maleic anhydride,2-(meth)acrylamido-2-methylpropanesulfonic acid,(meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone,N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycol(meth)acrylate, stearyl acrylate, and their salts.

(b) Crosslinking Agent (Inner Crosslinking Agent)

In the present invention, in terms of the water absorbent properties,use of a crosslinking agent (i.e.; inner crosslinking agent) isparticularly preferable. The crosslinking agent is used in an amount ofpreferably 0.001 to 5% by mole, more preferably 0.005 to 2% by mole,still more preferably 0.01 to 1% by mole, and particularly morepreferably 0.03 to 0.5% by mole to the monomer excluding thecrosslinking agent, in terms of physical aspect.

Examples usable as the crosslinking agent are one or more ofpolymerizable crosslinking agents (with polymerizable double bond of theacrylic acid), reactive crosslinking agents (with a carboxyl group ofthe monomer), and crosslinking agents having both of these properties.Concrete examples are, as a polymerizable crosslinking agent, compoundshaving at least two polymerizable double bonds in a molecule such asN,N-methylenebisacrylamide, (poly)ethylene glycol di(meth)acrylate,(polyoxyethylene)trimethylolpropane tri(meth)acrylate,poly(meth)allyloxyalkanes, etc. Further, examples of the reactivecrosslinking agent are covalent-binding crosslinking agents such aspolyglycidyl ether (ethylene glycol diglycidyl ether or the like), polyalcohols (propanediol, glycerin, sorbitol, etc.), and ion-bindingcrosslinking agents such as polyvalent metal compounds of aluminum orthe like. Among these crosslinking agents, in terms of water absorbentproperties, polymerizable crosslinking agents (with the acrylic acid),particularly, acrylate type, allyl type, and acrylamide typepolymerizable crosslinking agents are preferably used.

(c) Neutralizing Salt

Preferable examples as a basic substance to be used for neutralizationof the acrylic acid may include monovalent bases such as alkali metalhydroxides such as sodium hydroxide, potassium hydroxide, and lithiumhydroxide etc., and alkali metal (hydrogen) carbonates such as sodium(hydrogen) carbonate, potassium (hydrogen) carbonate, etc. Particularly,in terms of decrease of the residual monomers, neutralization into analkali metal acrylate especially with sodium hydroxide is preferable.The preferable conditions or the like in these neutralization treatmentsare exemplified in International Publication No. 2006/522181 and thedisclosed conditions are applicable for the present invention. Theneutralization temperature is preferably in a range of 10 to 100° C.,more preferably in a range of 30 to 90° C. The neutralizationtemperature is properly determined in this range, and a neutralizationmethod described below is preferable to decrease the residual monomers.

(d) Concentration of Monomer

Monomers may be polymerized generally in an aqueous solution. The solidcontent is generally 10 to 90 mass %, preferably 20 to 80 mass %, morepreferably 30 to 70 mass %, and particularly preferably 35 to 60 mass %.The polymerization may be carried out in the form of a slurry (waterdispersion liquid) exceeding the saturated concentration; however, interms of physical properties, it is carried out in an aqueous solutionwith the saturated concentration or lower.

(e) Other Monomer Components

The aqueous unsaturated monomer solution may contain a water-solubleresin or a water absorbent resin such as starch, polyacrylic acid(salt), or polyethylene imine, in combination with a monomer, in anamount of, for example, 0 to 50 mass %, preferably 0 to 20 mass %,particularly preferably 0 to 10 mass %, and most preferably 0 to 3 mass%. The solution may further contain a various kinds of foaming agents(carbonates, azo compounds, air bubbles, etc.), surfactants, oradditives described below, in an amount of, for example, 0 to 5 mass %and preferably 0 to 1 mass % to improve the various physical propertiesof a water absorbent resin or particulate water absorbent agent to beobtained. A graft polymer obtained by using other components (e.g.,starch-acrylic acid graft polymer) or a water absorbent resincomposition is also generically referred to as a polyacrylic acid(salt)-type water absorbent resin in the present invention.

As the additive, a chelating agent, hydroxycarboxylic acid, or areducing inorganic salt may be added, and it may be added to the waterabsorbent resin in such a manner that the amount thereof is preferably10 to 5000 ppm by mass, more preferably 10 to 1000 ppm by mass, stillmore preferably 50 to 1000 ppm by mass, and particularly preferably 100to 1000 ppm by mass. A chelating agent is preferable to be used.

The monomer is also preferable to contain a polymerization inhibitor.Examples of the polymerization inhibitor include such as methoxyphenoletc. and the content thereof is preferably 200 ppm or lower, morepreferably 10 to 160 ppm, and still more preferably 20 to 100 ppm(relative to monomer).

(f) Polymerization Step (Crosslinking Polymerization Step)

Owing to the performance and the easiness of polymerization control, thepolymerization method may be carried out by spray polymerization ordroplet polymerization, but preferably, in general, it is carried out byaqueous solution polymerization or reverse phase suspensionpolymerization. The aqueous solution polymerization is preferably whichare conventionally difficult to control polymerization or improve thecoloring, and further preferably continuous aqueous solutionpolymerization. An especially preferable controlling method is acontinuous polymerization method for producing the water absorbent resinin a huge scale of 0.5 t/h or higher, further 1 t/h or higher, stillmore 5 t/hr or higher, and still further 10 t/hr or higher bypolymerization of an aqueous unsaturated monomer solution in one line.Consequently, the preferable continuous polymerization may includemethods described as continuous kneader polymerization (e.g. U.S. Pat.Nos. 6,987,151 and 670141), continuous belt polymerization (e.g. U.S.Pat. Nos. 4,893,999 and 6,241,928, and US Patent Application PublicationNo. 2005/215734).

In addition, in the continuous polymerization, polymerization at a hightemperature starting (monomer at 30° C. or higher, 35° C. or higher,further 40° C. or higher, and particularly 50° C. or higher: the upperlimit is the boiling point), or a high monomer concentration (30 mass %or higher, 35 mass % or higher, further 40 mass % or higher, andparticularly 45 mass % or higher: the upper limit is the saturatedconcentration) can be exemplified as one preferable example.

The monomer stability is excellent in the present invention and thewater absorbent resin with white color can be obtained even by thepolymerization in such a high concentration and at such a hightemperature, and thus the effect is significantly exhibited in suchconditions. Preferable examples of high temperature initiatingpolymerization are described in U.S. Pat. Nos. 6,906,159 and 7,091,253etc. In the present invention, the monomer stability beforepolymerization is excellent and therefore, production in an industrialscale is made easy.

The polymerization can be carried out in atmospheric air; however, it ispreferable for coloring improvement to carry out the polymerization inan inert gas atmosphere of nitrogen or argon (e.g., oxygen concentrationof 1% by volume or lower) and also, the monomer is preferable to be usedfor polymerization after the dissolved oxygen in the monomer or thesolution containing the monomer is sufficiently replaced with an inertgas (e.g., less than 1 mg/L of dissolved oxygen). Even if such degassingis carried out, the monomer is excellent in the stability and thereforegelatinization before the polymerization does not occur and the waterabsorbent resin with higher physical properties and high whiteness canbe obtained.

(g) Polymerization Initiator

A polymerization initiator to be used for the present invention can beselected properly in accordance with the polymerization mode. Examplesof the polymerization initiator may include such as a photodecompositiontype polymerization initiator, a heat decomposition type polymerizationinitiator, and a redox type polymerization initiator. The amount of thepolymerization initiator may be 0.0001 to 1% by mole preferably and morepreferably 0.001 to 0.5% by mole to the monomer.

In the case of the amount of the polymerization initiator is large,coloring may possibly generate and in the case the of amount is low, itresults in increase of the residual monomer. Further, in the case of aconventional color-improve agent, it sometimes causes a negative effecton the polymerization; however, in the polymerization by the method ofthe invention, the coloring can be improved without causing any negativeeffect on the polymerization (such as previous time and the variousphysical properties) and therefore, it is preferable.

Examples of the photodecomposition type polymerization initiator mayinclude benzoin derivatives, benzyl derivatives, acetophenonederivatives, benzophenone derivatives, and azo compounds. Examples ofthe heat decomposition type polymerization initiator may includepersulfuric acid salts (sodium persulfate, potassium persulfate, andammonium persulfate), peroxides (hydrogen peroxide, tert-butyl peroxide,methyl ethyl ketone peroxide), azo compounds(2,2′-azobis(2-amindinopropane) dihydrochloride,2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, etc.). Amongthese radical polymerization initiators, persulfuric acid salts,peroxides, and azo compounds can be used as a photopolymerizationinitiator.

Examples of the redox type polymerization initiator may include theabove-mentioned persulfuric acid salts or peroxides in combination withreducing compounds such as L-ascorbic acid and sodium hydrogen sulfite.Further, combination use of a photodecomposition type initiator and aheat decomposition type polymerization initiator can also be exemplifiedas a preferable embodiment.

(2) Gel Shredding Step (Pulverization Step)

A hydrous gel-like crosslinked polymer obtained by polymerization(hereinafter, sometimes referred to as “hydrous gel”) may be dried as itis; however, it may be pulverized to be particulate (e.g., with a massaverage particle diameter of 0.1 to 5 mm, preferably 0.5 to 3 mm) duringpolymerization or after polymerization with a pulverizer (kneader, meatchopper, or the like) if necessary.

From the physical property aspect, regarding the temperature of thehydrogel at the time of gel pulverizing, the hydrogel is kept or heatedpreferably at 40 to 95° C. and more preferably 50 to 80° C. The resinsolid content of the hydrogel is not particularly limited; however, fromthe physical property aspect, it is preferably 10 to 70 mass %, morepreferably 15 to 65 mass %, and still more preferably 30 to 55 mass %.It is optional to add water, a polyhydric alcohol, a mixed liquid ofwater and a polyhydric alcohol, a solution obtained by dissolving apolyvalent metal in water, or their vapor, or the like. In the gelshredding step, a water absorbent resin fine powder or various kinds ofother additives may be kneaded.

(3) Drying Step

In order to accomplish a decrease in residual monomers, prevention ofgel deterioration (urea resistance), and prevention of yellowing in thepresent invention, the drying step is carried out via the gel shreddingstep after completion of the polymerization. The time until the start ofdrying via the gel shredding step is more preferable as it is shorter.That is, after being discharged out of the polymerization apparatus, ahydrous gel-like crosslinked polymer after polymerization starts to bedried preferably within 1 hour, more preferably within 0.5 hours, andstill more preferably within 0.1 hours (charged to a drier). In order toset the time within the range, shredding or drying is preferably carriedout directly without carrying out a storage step for the gel afterpolymerization. Further, to decrease the residual monomer and accomplishlow coloring, the temperature of the hydrous gel-like crosslinkedpolymer from completion of the polymerization to starting of the dryingis controlled preferably at 50 to 80° C. and more preferably at 60 to70° C.

The drying step provides a dried product having a resin solid content,which is calculated from a drying loss of the polymer (drying of 1 gpowder or particles at 180° C. for 3 hours) in an amount controlled tobe preferably 80 mass % or higher, more preferably 85 to 99 mass %,still more preferably 90 to 98 mass %, and particularly preferably 92 to97 mass %. The drying temperature is not particularly limited; however,it is preferably in a range of 100 to 300° C. and more preferably in arange of 150 to 250° C. To satisfy both of the high physical propertiesand whiteness, it is preferably that the drying temperature is 160 to235° C., more preferably 165 to 230° C. Further the drying time ispreferably within 50 minutes. If the temperature or the time is out ofthe above-mentioned range, it may possibly result in decrease of thewater absorption rate (CRC), increase of soluble matters (Extractables),and deterioration of whiteness index.

A various drying methods such as heat drying, hot-air drying, vacuumdrying, infrared drying, microwave drying, drying by a drum drier,azeotropic dehydration with a hydrophobic organic solvent, high humiditydrying using high temperature steam can be employed. It is preferablyhot-air drying with a gas with a dew point of preferably 40 to 100° C.and more preferably 50 to 90° C.

(4) Crushing and/or Classifying Step (Particle Size Adjustment AfterDrying)

After the drying step, the above-mentioned hydrous gel-like crosslinkedpolymer, the particle size may be adjusted after the drying ifnecessary. The polymer is preferably made to have a specified particlesize to improve the physical properties by surface crosslinkingdescribed below. The particle size can be adjusted properly bypolymerization (particularly reversed phase suspension polymerization),crushing, classification, granulation, and fine powder recovery.Hereinafter, the particle size is defined by a standard sieve (JISZ8801-1 (2000)).

The mass average particle diameter (D50) of the obtained water absorbentresin particles in the dried step before surface crosslinking isadjusted to be 200 to 600 μm, preferably 200 to 550 μm, more preferably250 to 500 μm, and particularly preferably 350 to 450 μm. It is morepreferable as the particles smaller than 150 μm are less, and theparticles are adjusted in a range of generally 0 to 5 mass %, preferably0 to 3 mass %, and more preferably 0 to 1 mass %. Further, it is morepreferable as the particles bigger than 850 μm are less, and theparticles are adjusted in a range of generally 0 to 5 mass %, preferably0 to 3 mass %, and more preferably 0 to 1 mass %. The logarithmicstandard deviation (σζ) of the particle size distribution is preferably0.20 to 0.40, more preferably 0.25 to 0.37, and particularly preferably0.27 to 0.35. Its measurement method may be a method described in, forexample, International Publication No. 2004/69915 and a method describedin EDANA-ERT 420.2-02 by using a standard sieve. The particle diameteris preferably applied also to the finally obtained water absorbent resinafter surface crosslinking.

In general if the particle size distribution is narrowed, that is, theupper and lower limits of the particle size are controlled to be narrow,the color becomes noticeable; however, the present invention is freefrom such color issue and is preferable. Accordingly, in the presentinvention, it is preferable to carry out a classification step to givethe ratio of particles with 150 to 850 μm of 90 mass % or more, morepreferably 95 mass % or more, particularly preferably 98 mass % (Theupper limit is 100 mass %) or more, after drying.

The bulk specific gravity of the water absorbent resin particles ispreferably 0.5 to 0.75 (g/cm³) and more preferably 0.6 to 0.7 (g/cm³). Ameasurement method thereof is described in detail in, for example, EDANAERT 460.2-02. In the case where the bulk specific gravity is notsatisfied, the stirring power index becomes difficult to be controlledor the physical properties may be lowered or powdering may be caused insome cases.

(5) Surface Treatment Step

The present invention provide a method for producing a polyacrylic acid(salt)-type water absorbent resin, comprising a step of preparing anaqueous monomer solution of an acrylic acid (salt), a step ofcontinuously polymerizing the aqueous monomer solution, a step of finelyshredding a hydrous gel-like crosslinked polymer during or afterpolymerization, a step of drying the obtained particulate hydrousgel-like crosslinked polymer, and a surface treatment step of adding andreacting a surface treatment agent to and with the dried water absorbentresin powder, wherein in the surface treatment step, a continuous mixingapparatus for the surface treatment agent and a continuous heatingapparatus are connected and periodical shielding is carried out betweenthe mixing apparatus and the heating apparatus.

As the continuous heating apparatus, it is preferable to use atransverse type continuous stirring apparatus having a charging inletand a discharge outlet for a water absorbent resin, as well as astirring means including one or more of rotary shafts equipped with aplurality of stirring disks and a heating means. At the time ofcrosslinking reaction, the stirring power index is preferable to set to3 to 15 W·hr/kg. Herein, the stirring power index is defined as(stirring power index)=((power consumption of apparatus at the time ofsurface treatment)−(power consumption at the time of idling)×averageretention time)/(treatment amount per unit time×average retention time),and a water absorbent resin with high physical properties can becontinuously and stably obtained even at the time of scale-up to a largescale (particularly, 1 t/hr or more) based on a specified apparatus andspecified parameters thereof (stirring power index).

The stirring power index can be easily calculated as described abovefrom the power consumption of apparatus at the time of surface treatmentand the power consumption at the time of idling. If this stirring powerindex exceeds 15 W·hr/kg, the physical properties (particularly, liquidpermeability) are deteriorated and on the other hand, if it is under 3W·hr/kg, the physical properties (particularly, water absorption againstpressure) are also deteriorated. The stirring power index is morepreferably in a range of 4 to 13 W·hr/kg, still more preferably 5 to 11W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably 5to 9 W·hr/kg.

The control of stirring power index can be determined properly inconsideration of adjustment of the supply amount and discharge amount ofthe water absorbent resin, the particle size or bulk specific gravity ofthe water absorbent resin, the rotation speed and shape of theapparatus, the composition of the surface treatment agent, and theretention time, and the preferable conditions will be described later.

Hereinafter, the preferable surface treatment method and control methodof stirring power index will be described.

(5-1) Humidifying and Mixing Step

This humidifying and mixing step is a step of adding and mixing asurface-crosslinking agent to and with the water absorbent resin powderobtained through the polymerization step to the classification step ifnecessary.

(a) Surface-Crosslinking Agent

The present invention further includes a surface-crosslinking step afterdrying. The production method of the present invention is preferablyapplicable for a method for producing a water absorbent resin with highabsorption against pressure (AAP) and liquid permeability (SFC) andcontinuous manufacture in a huge scale (particularly 1 t/hr), andparticularly preferably applicable for high temperaturesurface-crosslinking of a water absorbent resin.

Treatment agents described in Patent Documents 1 to 19, particularlysurface-crosslinking agents, are used for the surface treatment in thepresent invention. From the viewpoints of physical properties at thetime of scale-up, covalent bonding surface-crosslinking agents are usedamong them, and preferably covalent bonding surface-crosslinking agentsand ion bonding surface-crosslinking agents are used in combination.

(Covalent Bonding Surface-Crosslinking Agent)

Examples of a surface crosslinking agent to be employed in the presentinvention may include various organic or inorganic crosslinking agents,and organic surface crosslinking agents are preferably used. From theviewpoints of physical properties of obtained water absorbent resin,preferable examples to be used as the surface crosslinking agent arepolyhydric alcohol compounds, epoxy compounds, polyamine compounds andtheir condensation products with haloepoxy compounds, oxazolinecompounds (mono-, di-, or poly-)oxazolidinone compounds, and alkylenecarbonate compounds. Particularly dehydration reactive crosslinkingagents containing polyalcohol compounds, alkylene carbonate compounds,and oxazolidinone compounds, which require a high temperature reaction,are usable. In the case where no dehydration reactive crosslinking agentis used, the physical properties may sometimes be inferior or thedifference of the effects of the present invention may sometimes be hardto be caused in some cases.

More concretely, examples are compounds exemplified in U.S. Pat. Nos.6,228,930, 6,071,976, and 6,254,990. Examples are polyalcohol compoundssuch as mono-, di-, tri-, or tetra-propylene glycol, 1,3-propanediol,glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol,1,6-hexanediol, sorbitol, etc.; epoxy compounds such as ethylene glycoldiglycidyl ether, glycidol, etc.; alkylene carbonate compounds such asethylene carbonate; oxetane compounds; and cyclic urea compounds such as2-imidazolidinone.

(Ion-Bonding Surface Crosslinking Agent)

Further, other than the above-mentioned organic surface crosslinkingagent, an ion-bonding inorganic surface crosslinking agent (crosslinkingagent derived from polyvalent metal) may be used to improve the liquidpermeability potential or the like. Examples usable as the inorganicsurface crosslinking agent may include divalent or higher, preferably,trivalent to tetravalent polyvalent metal salts (organic salts andinorganic salts) and hydroxides. Polyvalent metals to be used arealuminum, zirconium, etc., and aluminum lactate and aluminum sulfate areusable. These inorganic surface crosslinking agents may be usedsimultaneously with or separately from the organic surface crosslinkingagent. The surface crosslinking with polyvalent metals is exemplified inInternational Publication Nos. 2007/121037, 2008/09843, and 2008/09842,in U.S. Pat. Nos. 7,157,141, 6,605,673, and 6,620,889, in US PatentApplication Publication Nos. 2005/0288182, 2005/0070671, 2007/0106013,and 2006/0073969.

Further, other than the above-mentioned organic surface crosslinkingagent, a polyamine polymer, particularly, having a mass averagemolecular weight of about 5000 to 1000000 may be used simultaneously orseparately to improve the liquid permeability potential and the like.Usable polyamine polymers are exemplified in U.S. Pat. No. 7,098,284,International Publication Nos. 2006/082188, 2006/082189, 2006/082197,2006/111402, 2006/111403, and 2006/111404 etc.

(The Use Amount)

The use amount of the surface crosslinking agent is preferably 0.001 to10 parts by mass and more preferably 0.01 to 5 parts by mass relative to100 parts by mass of the water absorbent resin particles. Water can bepreferably used in combination with the surface crosslinking agent. Theamount of water to be used is preferably in a range of 0.5 to 20 partsby mass and more preferably 0.5 to 10 parts by mass relative to 100parts by mass of the water absorbent resin particles. In the case ofusing the inorganic surface cross-linking agent and the organic surfacecrosslinking agent in combination, the agents are used preferably in arange of 0.001 to 10 parts by mass and more preferably 0.01 to 5 partsby mass, respectively.

Further, at that time, a hydrophilic organic solvent may be used and itsamount is in a range of 0 to 10 parts by mass and preferably 0 to 5parts by mass relative to 100 parts by mass of the water absorbent resinparticles. Still more, at the time of mixing a cross-linking agentsolution with the water absorbent resin particles, a water insolublefine particle powder, and a surfactant may coexist to an extent that theeffect of the present invention is not hindered, that is, in a range,for example, of 0 to 10 parts by mass, preferably 0 to 5 parts by mass,and more preferably 0 to 1 part by mass. The surfactant to be used andits use amount are exemplified in U.S. Pat. No. 7,473,739 etc.

(b) Mixing Apparatus

In the present invention, for mixing the surface treatment agent, acontinuous high speed rotation stirring type mixing apparatus,especially, a transverse type continuous high speed rotation stirringtype mixing apparatus (e.g., FIG. 4 and FIG. 5) is preferable used. Inaddition, the surface treatment agent refers to the above-mentionedsurface-crosslinking agent, or a substituent thereof (e.g., a radicalpolymerization initiator such as a persulfuric acid salt and a monomer)and is also a concept including a solution or dispersion liquid thereof.The stirring speed is preferably 100 to 10000 rpm and more preferably300 to 2000 rpm. The retention time is within 180 seconds, morepreferably 0.1 to 60 seconds, and particularly preferably 1 to 30seconds.

(c) Temperature of Water Absorbent Resin Before Surface-Crosslinking

In the present invention, the temperature of water absorbent resinparticles (e.g., a water absorbent resin in which it is mixed with thesurface-crosslinking agent and then before it is introduced into thesurface-crosslinking step; the resin is also referred to as aparticulate water absorbent agent) to be supplied to thesurface-crosslinking step or to a transportation tube is preferably 30°C. or higher, more preferably 40° C. or higher, and still morepreferably 50° C. or higher. The upper limit thereof is preferably 100°C. and more preferably 95° C. Deterioration of the physical propertiesof the water absorbent resin particles (particulate water absorbentagent) can be suppressed by keeping the temperature of the particulatewater absorbent resin to be supplied to a transportation tube at aprescribed temperature or higher. Specifically, a significant effect iscaused on maintain of the physical properties such as saline flowconductivity (SFC).

(5-2) Heat Treatment Step

This heat treatment step is a step of heating the wet mixture of a waterabsorbent resin powder and a surface treatment agent solution mixed inthe humidifying and mixing step to cause surface-crosslinking reaction.

(a) Structure of Heating Apparatus

(Inclined Angle)

Heat treatment is carried out for the water absorbent resin after thesurface treatment agent is added to the stirring apparatus. Thetransverse type continuous stirring apparatus is a necessary apparatus.In terms of control of the stirring power index, the transverse typecontinuous stirring apparatus is preferable to have a downward inclinedangle of 0.1 to 10°. The inclined angle is more preferably 0.5 to 5° andstill more preferably 1 to 4°. In the case where the inclined angle doesnot satisfy the range, the stirring power index becomes too high or toolow and the physical properties of the water absorbent resin maypossibly be deteriorated in some cases.

(Aspect Ratio)

The aspect ratio (length of apparatus in movement direction/width ofapparatus of cross section to movement direction) of the transverse typecontinuous stirring apparatus is preferably 1 to 20. The aspect ratio ismore preferably 1.5 to 10 and still more preferably 2 to 5. The aspectratio is determined as the ratio of the vertical (movement direction)length and the transverse (perpendicular to the movement direction in aplane) length in the inside of the apparatus. In the case where theaspect ratio does not satisfy the range, the stirring power indexbecomes too high or too low and the physical properties of the waterabsorbent resin may possibly be deteriorated in some cases or the pistonflow property in the apparatus may be sometimes worsened and thestability of the performance may be worsened in some cases.

(Scraping Blades)

The transverse type continuous stirring apparatus is preferable to havescraping blades (the scraping blades are denoted, for example, by 90a inFIG. 2). The scraping blades are described in Patent Document 31 (JP-ANo. 2004-352941). If the scraping blades are used, the stirring powerindex can be controlled to be low and as a result, the physicalproperties of the water absorbent resin can be improved.

(Average Retention Time)

In terms of control of the stirring power index within the preferablerange, the average retention time of the water absorbent resin ispreferably controlled to be 0.05 to 2 hours. The average retention timeis more preferably 0.1 to 1 hour and still more preferably 0.2 to 0.8hours.

The average retention time measurement for the water absorbent resin inthe transverse type continuous stirring apparatus of the presentinvention will be described. The retention time in the apparatus (alsoknown as heating time or reaction time in the transverse type continuousstirring apparatus) is controlled by various factors such as effectivevolume of the apparatus (in the case where there is a stirring shaftarranged in the transverse direction, the effective volume refers to thevolume covering the upper most surface of a stirring disk as theapex:the inner volume), the amount of the water absorbent resinparticles to be supplied, inclined angle, the rotation speed of thestirring shaft, the shape of the scraping blade, the bulk specificgravity of the water absorbent resin particles, the kind of the surfacetreatment agent, the height of the discharge bank installed in thedischarge outlet of the transverse type continuous stirring apparatus.These factors considerably affect not only the retention time but alsothe stirring power index. A method of measuring the average retentiontime is carried out by actually operating the apparatus under thecondition in which the various factors are fixed and measuring the massof the water absorbent resin particles remaining in the apparatus afterstopping the operation of the apparatus. Alternatively, the averageretention time can also be determined by unsteadily introducing asubstance easy to be identified (for example, a compound containingsulfur) into the charging inlet of the apparatus as a tracer substance,tracing the concentration fluctuation of the substance at the dischargeoutlet, obtaining the retention time distribution function, and carryingout calculation according to the retention time distribution function.As the tracer substance, for example, a water-soluble sulfuric acid saltcan be used. Further, as a concentration analysis method, there is amethod of tracing the concentration fluctuation by measuring theintensity ratio of the characteristic x-ray of sulfur and a monovalentcation (e.g., sodium) with EPMA, XMA, or the like in the case of apartially neutralized polyacrylic acid water absorbent resin. Theretention time distribution function and the average retention time aredescribed in detail in “Introduction of Chemical Reaction Engineering(Hannou Kogaku Gairon)”, Hiroshi KUBOTA, issued by the Nikkan KogyoShimbun, Ltd.

(Surface Roughness)

The inside of the transverse type continuous stirring apparatus ispreferably smooth and the surface roughness (Rz) thereof is controlledto be 800 nm or lower preferably. The surface roughness (Rz) ispreferably 500 nm or lower, more preferably 300 nm or lower, still morepreferably 200 nm or lower, particularly preferably 185 nm or lower, andmost preferably 170 nm or lower. In the case where the surface roughness(Rz) of the inside of the transverse type continuous stirring apparatusdoes not satisfy the range, since the friction resistance with the waterabsorbent resin particles becomes high, the stirring power index becomestoo high and the physical properties may possibly be deteriorated.

The surface roughness (Rz) means the value of the maximum height of thesurface unevenness and is defined according to JIS B 0601-2001. Thelower limit of the surface roughness (Rz) is 0 nm; however, there is notso much difference in the case where it is about 10 nm, and about 20 nmis satisfactory.

From the above-mentioned viewpoint, a material for the transverse typecontinuous stirring apparatus is preferably stainless steel and morepreferably obtained by mirror finishing. The mirror finishing suppressesdamage on the water absorbent resin powder. Examples of the stainlesssteel to be used for the apparatus include SUS 304, SUS 316, and SUS316L.

The surface roughness (Ra) other than the surface roughness (Rz) is alsodefined according to JIS B0601-2001 and a preferable value thereof isalso the same as that of the surface roughness (Rz). The surfaceroughness (Ra) is preferably 250 nm or lower and more preferably 200 nmor lower. These surface roughness values may be measured according toJIS B 0651-2001 by a probe type surface roughness meter. The surfaceroughness can be applied not only for the heating treatment apparatusbut also for apparatuses before and after the heating treatmentapparatus, preferably, for a cooling apparatus, a transportation pipe(particularly, a pneumatic transportation pipe) and a hopper, and theeffect of improving physical properties by surface-crosslinking can beheightened.

(Rotary Shaft and Stirring Disk)

The transverse type continuous stirring apparatus has one or a pluralityof rotary shafts, preferably 2 to 10 rotary shafts, and particularly 2shafts. Further, the number of a stirring disk (e.g., in FIG. 2) or astirring blade may be determined properly in accordance with the size(capacity) of the apparatus, and it is preferably 2 to 100 disks andmore preferably 5 to 50 disks per one shaft.

(Periodical Shielding)

In terms of physical property stability and improvement bysurface-crosslinking, it is to periodically shield the transverse typecontinuous high speed rotation stirring type mixing apparatus and thetransverse type stirring apparatus after the water absorbent resin andthe surface treatment agent solution are mixed to be introduced into thetransverse type stirring apparatus. The interval for the periodicalshielding is preferably 0.001 to 5 minutes, more preferably 0.005 to 1minute, still more preferably 0.01 to 0.1 minutes, and particularlypreferably 0.01 to 0.05 minutes. Execution of the periodical shieldingmakes it possible to carry out periodic introduction of the waterabsorbent resin into the continuous apparatus installed downstream(introduction of the water absorbent resin into a heating apparatus froma mixing apparatus or into a cooling apparatus from a heatingapparatus); that is, the introduction is turned on and offintermittently. In the case where no periodic shielding is carried outin the surface-crosslinking step, the physical properties of a waterabsorbent resin to be obtained may possibly be deteriorated in somecases. The shielding ratio (the ratio of the time when the waterabsorbent resin is shielded from the continuous apparatus installeddownstream) is preferably in a range of 1 to 80%, more preferably 2 to40%, still more preferably 5 to 30%, particularly preferably 5 to 20%,and most preferably 5 to 10%, in terms of stabilization of the physicalproperties (standard deviation). It is sufficient that the waterabsorbent resin in the above amount range (e.g., 1 t/hr or more) is fedto a next apparatus even if the periodical shielding is executed. Forexample, in the case of a rotary valve, the shielding interval isdefined as the reciprocal number (minute) of the rotation speed (rpm),and the shielding ratio is defined as a value calculated by dividing thetheoretical rotation speed (rpm) per one minute of the rotary valveneeded for discharging a mixture (wet powder; a mixture of the waterabsorbent resin and the surface-crosslinking agent solution) to besupplied to the continuous high speed mixing apparatus (theoreticalrotation speed is obtained from the volume flow rate calculated from thevolume per one rotation of the rotary valve, the mass flow rate of themixture to be discharged, and the bulk specific gravity) by actualrotation speed (rpm) of the rotary valve, and multiplying the calculatedvalue by 100. The shielding ratio is specifically defined as a valuecalculated by dividing the rotation speed (rpm) per one minute of therotary valve needed for discharging a wet powder (a mixture of the waterabsorbent resin and the surface-crosslinking agent) out of a mixingapparatus per unit time by the actual rotation speed of the rotaryvalve. It is calculated, for example, in the case of the example of thepresent invention as [1500×(1+(3.5/100))/0.47/1000/0.02/60/25]×100=11.0%.

The amount of the water absorbent resin retained by periodical shieldingis preferably 0 to 2 mass % and more preferably exceeding 0 and to 1mass % relative to the treatment amount. The volume per one rotation ofthe rotary valve may be determined properly and it is preferably 0.1 to0.001 [m³/lev (one rotation)], more preferably 0.2 to 0.002 [m³/lev],and still more preferably 0.1 to 0.01 [m³/lev]. In the case where theperiodic shielding is carried out or even when the periodic shielding isno carried out, when the continuous apparatuses (the mixing apparatus,the heating apparatus, and the cooling apparatus if necessary) areconnected, the distance from the outlet of an upstream apparatus and theinlet of an apparatus installed downstream is preferably 10 m orshorter. The distance is more preferably 5 m or shorter, still morepreferably 3 m or shorter, and particularly preferably 2 m or shorter.At the time of connecting these apparatuses, the apparatuses areconnected up and down; that is, the downstream apparatus is connected toa lower side of the upstream apparatus. A shielding apparatus of thewater absorbent resin particles may be installed between the upstreamapparatus and the downstream apparatus. The lower limit of the distancemay be determined properly in accordance with the sizes of theapparatuses or in a range in which a shielding apparatus described belowcan be housed. In the case where the distance is too large or theapparatuses are not connected up and down, the physical properties of awater absorbent resin to be obtained may possibly be deteriorated insome cases. In the case of connecting up and down, the mixing apparatus,the heating apparatus, and the cooling apparatus if necessary may beconnected up and down in this order. Connecting of the cooling apparatusmay be above or beside the heating apparatus.

A gate, a valve, a damper, a rotary feeder, a table feeder, or the likeis installed as a periodically shielding apparatus in a connecting partof the continuous apparatuses so that the periodical shielding can becarried out. Examples of the gate to be employed include a slide gate, aroller gate, a tainter gate, a radial gate, a flap gate, a rolling gate,and a rubber gate etc. Examples of the valve to be employed include aHowell-Bunger (fixed cone dispersion) valve, a hollow jet valve (amovable cone dispersion valve), a jet flow valve, a butterfly valve, agate valve (a partition valve), an orifice valve, a rotary valve (avalve for opening or closing by rotating a cylinder), and a Johnsonvalve (a valve for opening or closing by moving a conical valve bodyback and forth). These shielding apparatuses may be installed in anoutlet of the continuous mixing apparatus (e.g., FIG. 4 and FIG. 5), orin an inlet of the continuous heating apparatus (e.g., FIG. 1), or in amiddle part thereof after the outlet of the mixing apparatus and theinlet of the heating apparatus are connected. The periodic shielding isalso similarly carried out preferably in an outlet of the heatingapparatus, consequently the periodic shielding is carried out between anoutlet of the heating apparatus (e.g., FIG. 1) and the cooling apparatus(e.g., FIG. 3). The shielding and connecting of the apparatuses arepreferably carried out via a valve, particularly a rotary valve, amongthese shielding apparatuses. The size (it refers to diameter: however,in the case where the cross section is other than a circular shape, itis converted into the diameter of a circle with the same surface area)of the valve may be selected properly and it is preferably, for example,1 to 100 cm in diameter and more preferably 10 to 50 cm in diameter.

Each shielding apparatus is operated at less than 100% of the maximumtreatment amount (kg/hr; the maximum amount of a substance which can bepassed through the shielding apparatus per unit time). The operationcondition is preferably 5 to 95%, more preferably 10 to 90%, and stillmore preferably 20 to 80%. In the case where the operation condition ofthe shielding apparatus is out of the above-mentioned range, thephysical properties of a water absorbent resin to be obtained maypossibly be deteriorated in some cases and the performance may possiblybecome unstable. In the case where a rotary shielding apparatus such asa rotary valve is used, the rotation speed thereof may be determinedproperly and it is preferably 1 to 500 rpm (rotation/minute). Therotation speed is more preferably 5 to 200 rpm, still more preferably 10to 100 rpm, and particularly preferably 20 to 100 rpm. The maximumtreatment performance of the shielding apparatus may be determinedproperly and it is, for example, preferably 0.01 to 20 t/hr and morepreferably 0.1 to 5 t/hr.

(b) Operation Condition of Heating Apparatus

After the surface treatment agent is added to the stirring type mixingapparatus and the water absorbent resin and the surface treatment agentsolution are mixed, heat surface-crosslinking treatment is carried out.The transverse type continuous stirring apparatus is an apparatusnecessary for heating treatment. The water absorbent resin is treated byheat treatment and is also treated by second heat treatment ifnecessary, and thereafter is treated by cooling treatment. The heatingtemperature (heat transfer surface temperature of a jacket or the like)is 70 to 300° C., preferably 120 to 250° C., and more preferably 150 to250° C., and the heating time is preferably in a range of 1 minute to 2hours. The heat treatment is carried out generally by a drier or aheating furnace. The present invention can provide a water absorbentresin with high whiteness even by high temperature heating or dryingwith air (hot blow) which conventionally causes intense coloration.

(Filling Ratio)

It is preferable to continuously supply the water absorbent resin insuch a manner that the filling ratio (volume ratio) of the transversetype continuous stirring apparatus with the water absorbent resin can be50 to 90%. The filling ratio is more preferably 55 to 85% and still morepreferably 60 to 80%. In the case where the filling ratio does notsatisfy the above-mentioned range, the stirring power index is hard tobe controlled, and the physical properties of a water absorbent resin tobe obtained may possibly be deteriorated in some cases. The position at100% filling ratio is the apex part of a stirring disk of a rotary shaftas described above.

It is preferable to continuously supply the water absorbent resin insuch a manner that the mass surface area ratio of the water absorbentresin in the transverse type continuous stirring apparatus can be 100kg/m²/hr or lower. It is more preferably 90 kg/m²/hr or lower and stillmore preferably 50 to 70 kg/m²/hr. In the case where the mass surfacearea ratio does not satisfy the above-mentioned range, the stirringpower index is hard to be controlled and the physical properties of awater absorbent resin to be obtained may possibly be deteriorated insome cases.

Herein, the mass surface area ratio is defined by the followingequation.

(Mass surface area ratio)=(Mass flow rate of water absorbent resin perunit time)/(Heat transfer area of apparatus)

In the case where the jacket surface of an apparatus trough is only forheat insulation, the mass surface area ratio is defined as follows.

(Mass surface area ratio)=(Mass flow rate of water absorbent resin perunit time)/(Heat transfer area of stirring shaft and stirring disk ofapparatus)

(Rotation Speed and Reaction Time)

According to the present invention, uniform heating and mixing can becarried out by adjusting the stirring speed of the transverse typecontinuous stirring apparatus to 2 to 40 rpm. If it is lower than 2 rpm,the stirring becomes insufficient and on the other hand, if it is higherthan 40 rpm, a fine powder tends to be generated easily in some cases.The stirring speed is more preferably 5 to 30 rpm. The retention time inthe apparatus is, for example, 10 to 180 minutes and more preferably 20to 120 minutes. If it is shorter than 10 minutes, the crosslinkingreaction tends to be insufficient. On the other hand, if it exceeds 180minutes, the water absorption performance may possibly be deterioratedin some cases.

(Pressure Reduction)

In the present invention, it is preferable to set the inside of thetransverse type continuous stirring apparatus to slightly reducedpressure. “Pressure-reduced state” means barometric pressure lower thanatmospheric pressure. In addition, “degree of pressure reductionrelative to atmospheric pressure” means the pressure difference with theatmospheric pressure and is denoted as a positive (plus) value in thecase where barometric pressure is lower than atmospheric pressure. Forexample, in the case where atmospheric pressure is standard atmosphericpressure (101.3 kPa), the expression that “degree of pressure reductionis 10 kPa” means that barometric pressure is 91.3 kPa. In thisapplication, “degree of pressure reduction relative to atmosphericpressure” may also be referred to simply as “degree of pressurereduction”. In the case where pressure is not reduced, a water absorbentresin powder may possibly flow over the air intake port of the mixingapparatus and it is thus not preferable. Dust (ultrafine particles ofthe water absorbent resin or inorganic fine particles used if necessary)is removed from the water absorbent resin by slightly reducing thepressure and thus it is also preferable in terms of a decrease in dust.

From the viewpoint of improvement of the effect attributed to pressurereduction, the lower limit of the degree of pressure reduction ispreferably higher than 0 kPa, more preferably 0.01 kPa or higher, andstill more preferably 0.05 kPa or higher. Excess pressure reduction maypossibly remove even a necessary water absorbent resin powder besidesdust to the outside of the apparatus and it may possibly result in adecrease in yield. Additionally, from the viewpoints of suppression ofleap of a powder in the system and of suppression of excess cost for theexhaust system, the degree of pressure reduction is preferably 10 kPa orlower, more preferably 8 kPa or lower, still more preferably 5 kPa orlower, and particularly preferably 2 kPa or lower. The preferablenumeral range of the degree of pressure reduction may be selectedarbitrarily between the lower limit and the upper limit.

(Atmosphere)

The atmosphere in the transverse type continuous stirring apparatus maybe air, or an inert gas such as nitrogen for prevention of coloration orprevention of combustion, and steam may be added properly. Thetemperature and the dew point are determined properly, and theatmospheric temperature (defined as the gas temperature in the upperpart space of the apparatus) is preferably 30 to 200° C. and morepreferably 50 to 150° C. The dew point is preferably 0 to 100° C. andmore preferably 10 to 80° C.

(5-3) Cooling Step

The cooling step is a step carried out arbitrarily after the heatingtreatment step. In the case where a dehydration reactive crosslinkingagent such as a polyhydric alcohol compound, an alkylene carbonatecompound, and an oxazolidinone compound is used as thesurface-crosslinking agent, it is preferable to carry out the coolingstep.

A cooling apparatus used in this cooling step is not particularlylimited and a transverse type continuous stirring apparatus to be usedin the above-mentioned heating treatment may be used and also, a biaxialstirring and drying apparatus in which cooling water is circulated inthe inside of the inner wall and other heat transmission faces and whichis exemplified in the Patent Document 41 (U.S. Pat. No. 7,378,453) maybe used. The temperature of the cooling water is controlled to be lowerthan the heating temperature in the surface treatment step andpreferably 25° C. or higher and lower than 80° C. In the presentinvention, the surface treatment reaction by heating can be controlledby a cooling apparatus arbitrarily installed and the physical propertiesof the water absorbent resin can be improved. Examples of the coolingapparatus to be suitably used may include cooling apparatusesexemplified in Patent Document 41 for mechanical stirring (optionallycombined with stirring by gas current) and also cooling apparatuses forstirring and mixing by stirring by vibrations and stirring by gascurrent in combination. Herein, it is preferable to carry out theperiodic shielding for the inlet of the cooling apparatus (connected tothe outlet of the heating apparatus) and further for the outlet of thecooling apparatus.

The cooling step is carried out preferably in the connected transversetype continuous stirring apparatus (e.g., FIG. 3). The stirring powerindex of the cooling apparatus is preferable 3 to 15 W·hr/kg, morepreferably 4 to 13 W·hr/kg, still more preferably 5 to 11 W·hr/kg,particularly preferably 5 to 10 W·hr/kg, and most preferably 5 to 9W·hr/kg. The pressure reducing is similarly carried out preferably inthe case of the heating step, and the periodic shielding is alsosimilarly carried out preferably in the case of the heating apparatus.Herein, the stirring power index (4 to 13 W·hr/kg, more preferably 5 to11 W·hr/kg, particularly preferably 5 to 10 W·hr/kg, and most preferably5 to 9 W·hr/kg) of the heating apparatus described above (also known asheat treatment apparatus, heater) may be the same or different from thatof the cooling apparatus; however, in terms of physical properties, thestirring power index of the cooling apparatus (also known as chiller) ispreferable to be smaller. The stirring power index of the coolingapparatus is preferably in a range of 0.99 to 0.25 times, morepreferably 0.95 to 0.50 times, and particularly preferably 0.90 to 0.55times as high as that of the heating apparatus.

(5-4) Others

(a) Number of Surface Treatment Apparatuses

In terms of improvement of the stirring power index and physicalproperties, the polymerization step may be carried out preferably bycontinuous belt polymerization or continuous kneader polymerization anda plurality of surface treatment steps are preferably carried out inparallel for the polymerization step.

In the production method of the present invention, in terms ofimprovement of physical properties and stabilization, thesurface-crosslinking step is carried out in 2 or more lines for 1 lineof the polymerization step. The 1 line in the present invention meansone system in which steps proceed from a raw material (monomer) untilwhen a polymer gel, a water absorbent resin (including a recovered finepowder product), a particulate water absorbent agent and a final productare obtained. In the case where the system is branched into two, it isreferred to as “2 lines”. In other words, “2 or more lines” means a modein which two or more apparatuses are arranged in parallel and operatedsimultaneously or alternately in a single step.

In the present invention, in the case where the respective steps arecarried out in 2 or more lines, the upper limit for each step is about10 lines, especially preferably 2 to 4 lines, still more preferably 2 to3 lines, and particularly preferably 2 lines. The physical properties ofa water absorbent resin to be obtained are improved by adjusting thenumber of the lines within the above range. From a viewpoint that in thecase where the number of lines (divisions) is large, no dividing effectis caused, and the operation becomes complicated, and also it is noteconomical in terms of the cost, it is particularly preferable tosimultaneously operate 2 lines, that is, 2 or more of the sameapparatuses (particularly two apparatuses) in parallel.

In the present invention, the polymer gel or the water absorbent resin,which is a dried product of the polymer gel, is divided into 2 or morelines in the steps after the drying step, and the ratio of amountsdivided may be determined for every step without any particularlimitation. For example, in the case of dividing into 2 lines, the ratiois preferably 4:6 to 6:4, more preferably 4.5:5.5 to 5.5:4.5, still morepreferably 4.8:5.2 to 5.2:4.8, and most preferably 5:5. Even in the caseof dividing into 3 or more lines, it is preferable that the ratio of themaximum amount and the minimum amount of n divided portions is withinthe above range. The dividing operation may be carried out in acontinuous manner or in a batch manner and the ratio of amounts dividedis defined in accordance with the average amounts for a prescribed time.

In the present invention, the number of lines in thesurface-crosslinking step is not particularly limited and the numberthereof may be selected arbitrarily; however, in consideration of theconstruction cost and running cost of a plant etc., 1 line or 2 linesare preferable and 2 lines are particularly preferable. That is, interms of physical properties, it is most preferable that thesurface-crosslinking step and preferably further the crushing step andthe classification step all have 2 or more lines (the upper limit iswithin the above range) for 1 line of the polymerization step.

In addition, in the case where a plurality of apparatuses are installedin parallel in place of one apparatus in the present invention, theapparatuses in parallel may properly be miniaturized. Even if thetreatment capacity of the apparatus is miniaturized into ½, the cost ofthe apparatus is not lowered to a half; however, in the presentinvention, installation of specified apparatuses in parallel improvesthe physical properties of an absorbent agent to be obtained anddecreases the ratio of the product out of the specification and it isthus found that it consequently results in a decrease in cost.

US Patent Application Publication No. 2008/0227932 discloses a techniqueof carrying out “polymerization in 2 lines” and the latter half in oneline; Patent Document 30 (US Patent Application Publication No.2007/149760) discloses a technique of “connecting in series” of astirring and drying apparatus and a heating apparatus forsurface-crosslinking; and WO 2009/001954 discloses a technique of“connecting in series” of belt polymerization apparatuses. In contrast,in the present invention, the physical properties are improved andstabilization is accomplished more than before by “arranging(substantially the same) apparatuses in parallel” in the specified stepafter completion of the polymerization step for one polymerizationapparatus.

(Dividing Means)

In order to carry out surface-crosslinking in 2 or more lines in thepresent invention, a dividing step is included, and a dividing step of aparticulate hydrous gel or a particulate water absorbent resin, which isa dried product of the gel, is preferably included, and a dividing stepof a particulate water absorbent resin is more preferably included.

A dividing method to be employed may be, for example, the followingmeans (a-1) to (a-3) for the particulate water absorbent resin afterdrying.

(a-1) A method for dividing the particulate water absorbent resin afterstorage in a hopper. Preferably, a quantitative feeder for a powder isused. As the quantitative feeder, a cycle feeder or a screw feeder isused suitably.

(a-2) A method for dividing the particulate water absorbent resin duringthe time of pneumatic transportation to a plurality of hoppers.

(a-3) A method for dividing the particulate water absorbent resin at thetime of dropping (e.g., free fall). In this case, a riffle divider, a3-way divider, or the like having hills and dams are used for thedividing. Additionally, a JIS sample reducing and dividing apparatus(riffle divider) has a structure partitioned into a large number ofsmall chambers in which a fed sample is distributed alternately to twodirections.

A dividing method to be employed for the polymer gel afterpolymerization may be, for example, the following means (a-4) to (a-6)or the combination thereof, and the polymer gel is supplied to thedrying step in parallel.

(a-4) A method for dividing the particulate hydrous gel obtained by akneader or a meat chopper at the time of dropping (e.g., free fall). Forthe dividing, a riffle divider, a 3-way divider, or the like havinghills and dams are used in the outlet of the kneader or the meatchopper.

(a-5) A method for dividing the particulate hydrous gel by aquantitative feeder.

(a-6) A method for cutting a sheet-like gel obtained by beltpolymerization.

Among them, it is preferable that at least the particulate waterabsorbent resin after drying is divided and in order for that, thepolymerization gel or the particulate dried product is divided.

A preferable value of the dividing ratio of the particulate waterabsorbent resin and the polymerization gel to be divided in the mode isas described above.

Among them, the means (a-1) to (a-3) are preferably employed and themeans (a-1) is more preferably employed in terms of the quantitativesupplying property.

(b) Hopper

In terms of the surface-crosslinking property in the present invention,a hopper is preferable to be used before and after thesurface-crosslinking. The hopper to be employed is more preferably ahopper with an inverse truncated pyramidal shape, an inverse circulartruncated conical shape, a shape formed by adding a square pillar withthe same shape as the maximum diameter part of an inverse truncatedpyramid to the pyramid, a shape formed by adding a cylindrical columnwith the same shape as the maximum diameter part of an inverse circulartruncated cone to the cone. The material thereof is not particularlylimited; however, a hopper made of stainless steel is preferable to beemployed and the surface roughness thereof is within the above rangepreferably. A preferable hopper and the shape thereof are exemplified inPCT/JP 2009/54903 and such a hopper is recommended.

(c) Transportation of Water Absorbent Resin Before and AfterSurface-Crosslinking

Various kinds of methods may be employed as a method of transporting thewater absorbent resin before and after surface-crosslinking, andpreferably pneumatic transportation is employed. From a viewpoint thatthe excellent physical properties of the water absorbent resin particlesand/or water absorbent resin powder can be maintained stably and theobstruction phenomenon can be suppressed, dried air is preferable to beemployed as primary air and secondary air used based on the necessity(additional air for pneumatic transportation). The dew point of the airis generally −5° C. or lower, preferably −10° C. or lower, morepreferably −12° C. or lower, and particularly preferably −15° C. orlower. The range of the dew point is −100° C. or higher, preferably −70°C. or higher, and it is sufficient about −50° C. in consideration ofcost. Further, the temperature of the gas is 10 to 40° C. and morepreferably about 15 to 35° C. The adjustment of the dew point ofcompressed air to be used at the time of pneumatic transportation to theabove range can suppress a decrease in SFC, especially, at the time ofwrapping the water absorbent resin as a product and thus, it ispreferable.

Besides the use of the dried gas (air), heated gas (air) may be used. Aheating method is not particularly limited and a gas (air) may bedirectly heated using a heat source or the transportation part or pipemay be heated to indirectly heat a gas (air) flowing therein. Thetemperature of the heated gas (air) is preferably 20° C. or higher as alower limit and more preferably 30° C. or higher, and preferably lowerthan 70° C. as a upper limit and more preferably lower than 50° C.

A method of controlling the dew point, a gas, preferably air, may bedried properly. Specifically, examples thereof include a method using amembrane drier, a method using a cooling and adsorption type drier, amethod using a diaphragm drier, or a method using these methods incombination. In the case where an adsorption type drier is used, it maybe of thermal regeneration manner, a non-thermal regeneration manner, ora non-regeneration manner.

(6) Other Steps

Besides the above-mentioned steps, as required, a recycling step of theevaporated monomer, a granulating step, a fine powder removing step, afine powder recycling step, etc. may be added. Further, in order toexhibit the effect of color stabilization over time, prevent geldeterioration, or the like, an additive described below may be used forthe monomer or the polymer thereof.

[3] Polyacrylic Acid (Salt)-Type Water Absorbent Resin

(1) Physical Properties of Polyacrylic Acid (Salt)-Type Water AbsorbentResin

In the case the purpose is to use the polyacrylic acid (salt)-type waterabsorbent resin of the present invention for a sanitary material,particularly a paper diaper, it is preferable to control at least one ofthe following (a) to (e), further two or more including AAP, and stillmore three or more by the above-mentioned polymerization andsurface-crosslinking. In the case the followings are not satisfied, thewater absorbent resin sometimes fails to exhibit sufficient function inform of a high concentration diaper described below. The productionmethod of the present invention is more effective for producing a waterabsorbent resin attaining the following physical properties andparticularly for stabilizing the physical properties (narrowing thestandard deviation). That is, among the following physical properties ofinterest, the production method of the prevent invention is preferablyapplied to a method for producing a water absorbent resin having a waterabsorption against pressure (AAP) of 20 g/g or higher for an aqueous 0.9mass % sodium chloride solution at a pressure of 4.8 kPa, a 0.69 mass %physiological saline flow conductivity (SFC) of 1 (×10⁻⁷·cm³·s·g⁻¹) orhigher, and a water absorption under no pressure (CRC) of 20 g/g orhigher, and more preferably applied to a production method within thefollowing range to improve or stabilize the physical properties.

(a) Water Absorption Against Pressure (AAP)

In order to prevent leakage in a diaper, the water absorption againstpressure (AAP) for an aqueous 0.9 mass % sodium chloride solution undera pressure of 1.9 kPa and that of 4.8 kPa is controlled to be preferably20 g/g or higher, more preferably 22 g/g or higher, and still morepreferably 24 g/g or higher as one example of means for accomplishingthe surface-crosslinking and the cooling step carried out thereafter.The AAP is more preferable as it is higher; however, in terms of thebalance between other physical properties and cost, the upper limit ofthe AAP is about 40 g/g at 1.9 kPa and about 30 g/g at 4.8 kPa. The AAPis shown as a value at 4.8 kPa unless otherwise specified.

(b) Liquid Permeability (SFC)

In some cases, in order to prevent a leakage from a diaper, the liquidpermeability under pressure, which is a flow conductivity SFC (definedin U.S. Pat. No. 5,669,894) to a 0.69% physiological saline flowconductivity (SFC) is controlled to be 1 (×10⁻⁷·cm³·s·g⁻¹) or higher,preferably 25 (×10⁻7·cm³·s·g⁻¹) or higher, more preferably 50(×10⁻⁷·cm³·s·g⁻¹) or higher, still more preferably 70 (×10⁻⁷·cm³·s·g⁻¹)or higher, and particularly preferably 100 (×10⁻⁷·cm³·s·g⁻¹) or higheras one example of means for accomplishing the surface-crosslinking andthe cooling step carried out thereafter.

In order to more effectively improve the liquid permeability, especiallyto improve SFC to 25 (×10⁻⁷·cm³·s·g⁻¹) or higher, the present inventionis preferably applied for producing a water absorbent resin with highliquid permeability.

(c) Water Absorption Under No Pressure (CRC)

Water absorption under no pressure (CRC) is controlled to be preferably10 (g/g) or higher, more preferably 20 (g/g) or higher, still morepreferably 25 (g/g) or higher, and particularly preferably 30 (g/g) orhigher. The CRC is more preferable as it is higher, and the upper limitis not particularly limited; however, in consideration of balance withother physical properties, it is preferably 50 (g/g) or lower, morepreferably 45 (g/g) or lower, and still more preferably 40 (g/g) orlower.

(d) Amount of Water Soluble Components (Dissolve Amount)

The amount of water soluble components is preferably 0 to 35 mass % orlower, more preferably 25 mass % or lower, still more preferably 15 mass% or lower, and particularly preferably 10 mass % or lower.

(e) Residual Monomer

Using the above-mentioned polymerization as one example of achievingmeans, the amount of the residual monomer is adjusted to be generally500 ppm by mass or lower, preferably 0 to 400 ppm by mass, morepreferably 0 to 300 ppm by mass, and particularly preferably 0 to 200ppm by mass.

(2) Other Additives

Further, in accordance with the purpose, 0 to 3 mass % and preferably 0to 1 mass % of an oxidizing agent, an antioxidant, water, a polyvalentmetal compound, a water-insoluble inorganic or organic powder such assilica and metal soap, etc. as well as a deodorant, an antibacterialagent, a polymer polyamine, pulp, and thermoplastic fibers, etc. may beadded to the water absorbent resin.

(3) Purpose of Use

The purpose of use of the polyacrylic acid (salt)-type water absorbentresin of the present invention is not particularly limited; however, itis preferable to use the water absorbent resin for an absorbing articlesuch as a paper diaper, a sanitary napkin, an incontinence pad, or thelike. Particularly, the water absorbent resin exhibits excellentperformance in case of being used in a high-consistency diaper (onediaper in which a large amount of the water absorbent resin is used)that conventionally has a problem of malodor and coloring derived fromraw materials and particularly in the case of being used in a top layerpart of an absorbent body of the absorbing article.

The content (core concentration) of the water absorbent resin in theabsorbent body which may contain arbitrarily other absorbing materials(pul_(p) fibers or the like) in the absorbing article is 30 to 100 mass%, preferably 40 to 100 mass %, more preferably 50 to 100 mass %, stillmore preferably 60 to 100 mass %, particularly preferably 70 to 100 mass%, and most preferably 75 to 95 mass % to exhibit the effect of thepresent invention. For example, in the case where the water absorbentresin of the present invention is used especially for an upper layerpart of an absorbent body in the above concentration, the waterabsorbent resin is excellent in the dispersion property of an absorbedliquid such as urine or the like owing to the high liquid permeability(liquid permeability against pressure) and therefore, an absorbentproduct such as paper diaper can efficiently distribute a liquid andimprove the amount absorbed by the whole of the absorbent product.Additionally, since the absorbent body keeps highly advanced whitecolor, an absorbent product with sanitary impression can be provided.

EXAMPLES

Hereinafter, the effects of the present invention will be made apparentby way of examples; however, the present invention should not beconstrued in a limited way based on the description of the examples. Inaddition, measurement methods for AAP, SFC, and the like in thefollowing description are as described above.

Production Example 1 Production of Water Absorbent Resin Particle (A)

A continuous production apparatus for a polyacrylic acid (salt)-typewater absorbent resin was used which was obtained by connectingrespective apparatuses for a polymerization step (static polymerizationon a belt), a gel shredding step (pulverization step), a drying step, acrushing step, a classification step, and a transportation step betweenthe respective steps and which could carry out the respective stepscontinuously. The production capacity of this continuous productionapparatus was about 1500 kg per an hour. Water absorbent resin particleswere continuously produced by using this continuous productionapparatus.

First, an aqueous acrylic acid sodium salt solution partiallyneutralized at 75 mol % was prepared as an aqueous monomer solution (1).The aqueous monomer solution (1) contained 0.06 mol % of polyethyleneglycol diacrylate (average n number=9) as an inner crosslinking agentrelative to the total mole number. The concentration of the monomer(partially neutralized acrylic acid sodium salt) in the aqueous monomersolution (1) was 38 mass %. The obtained aqueous monomer solution (1)was continuously fed onto a belt by a constant rate pump. Nitrogen gaswas continuously blown in the middle of the pipe used for the feedingand the oxygen concentration dissolved in the aqueous monomer solution(1) was adjusted to 0.5 mg/L or lower. In addition, the above-mentioned“average n number” means the average number of degree of methylene chainpolymerization in the polyethylene glycol chain.

Next, sodium persulfate and L-ascorbic acid were continuously mixed byline mixing with the aqueous monomer solution (1). In this line mixing,the mixing ratio of the sodium persulfate was adjusted to 0.12 g per onemole of the monomer and the mixing ratio of the L-ascorbic acid wasadjusted to 0.005 g per one mole of the monomer. The continuously mixedmaterial obtained by line mixing was supplied in a thickness of about 30mm to a flat plane steel belt having banks in both ends and subjectedcontinuously to static aqueous solution polymerization for about 30minutes to obtain a hydrous gel-like crosslinked polymer (1). Thehydrous gel-like crosslinked polymer (1) was finely shredded into about2 mm by a meat chopper with a hole diameter of 7 mm, spread on a movableporous plate of a continuous ventilating band drier in such a mannerthat the thickness thereof was adjusted to 50 mm, and dried at 185° C.for 30 minutes to obtain a dried polymer. Herein, the time taken fromthe outlet of the polymerization apparatus to the inlet of the drier waswithin 1 minute. The entire amount of the dried polymer was continuouslysupplied to a three-step roll mill, followed by crushing. The roll gapsof the three-step roll mill were 1.0 mm/0.55 mm/0.42 mm in this orderfrom the upper side. After the crushing, the crushed polymer wasclassified by a sieving apparatus having metal sieving nets with 850 μmmeshes and 150 μm meshes to obtain water absorbent resin particles (A)containing about 98 mass % of particles having a particle diameter of150 to 850 μm. The CRC of the water absorbent resin particles (A) was 35g/g and the bulk specific gravity thereof was 0.6 g/cm³.

Example 1

A water absorbent resin powder (1) was produced by using a continuousproduction apparatus involving a surface treatment step (wetting andmixing step, heating step, and cooling step), a particle regulationstep, and a transportation step connecting the respective steps,successively from the continuous production apparatus used in ProductionExample 1. That is, the classification step of Production Example 1 andthe surface treatment step in Example 1 were connected by thetransportation step. The high speed continuous mixing apparatus and thetransverse type continuous stirring apparatus are connected to eachother up and down at a distance of 1.5 m.

The water absorbent resin particles (A) were pneumatically transportedto a temporarily storing hopper by pneumatic transportation (temperature35° C. and dew point −15° C.) from the classifying apparatus andcontinuously supplied at 1.5 t/hr via a constant rate feeder by a highspeed continuous mixing apparatus (Turbulizer/1000 rpm) and at the sametime a surface treatment agent solution (1) was mixed by spraying(wetting and mixing step). The surface treatment agent solution (1) wasa mixed solution containing 1,4-butanediol, propylene glycol, and purewater. The surface treatment agent solution (1) was mixed with the waterabsorbent resin particles (A) at a ratio of 0.3 parts by mass of1,4-butanediol, 0.5 parts by mass of propylene glycol, and 2.7 parts bymass of pure water relative to 100 parts by mass of the water absorbentresin particles (1) to give a mixture (1), a wet powder. The bulkspecific gravity of the wet powder was 0.47 g/cm³.

The obtained mixture (1) was then surface-treated by a transverse typecontinuous stirring apparatus (1) having a downward inclined angle of1°, an aspect ratio of 2.2, a paddle rotation speed of 13 rpm, tworotary shafts, and stirring disks having scraping blades, and a surfaceroughness (Rz) of the inner surface of 500 nm (heat treatment step). Atthat time, the inside of the apparatus (1) was suctioned by a suctioninggas discharge apparatus having a bag filter, and the inside pressure ofthe apparatus was reduced to 1 kPa. A rotary valve (periodicallyshielding apparatus) was installed in the inlet (connecting part withthe mixing apparatus) and outlet (connecting Part with the coolingapparatus) of the apparatus (1). The volume per one rotation of a rotaryvalve was 0.02 m³/lev, the rotation speed was 25 rpm, the shieldinginterval was 0.04 min (defined as the reciprocal number of the rotationspeed, and thus it was 1/25=0.04 min because of 25 rpm), and theshielding ratio was 11.0%. In relation to this, the shielding ratio wasa value calculated by dividing the rotation speed (rpm) per one minuteof a rotary valve required for discharging the mixture (1), which was awet powder, discharged out of the high speed continuous mixing apparatusper unit time by the actual rotation speed of the rotary valve. It wascalculated in this Example as[1500(1+(3.5/100))/0.47/1000/0.02/60/25]×100=11.0%.

According to a previous test, the position of a discharge bank whichgave an average retention time of about 45 minutes and an averagefilling ratio of 75% was measured, and the discharge bank was set at theposition as measured. A heating source used for the surface treatmentwas pressurized steam at 2.5 MPa, and the temperature of the mixture (1)in the apparatus was measured by a thermometer installed near thedischarge part of the transverse type continuous stirring apparatus (1),and the steam flow rate was controlled to carry out the heating in sucha manner that the temperature was adjusted to 198° C. The total surfacearea of the stirring disks and the stirring shafts was 24.4 m² and themass surface area ratio calculated from the total surface area (heattransfer surface area) and the treatment amount was 61.5 kg/m²/hr. Thestirring power at the time of the surface treatment was 27.8 kW, thestirring power in idling was 13.5 kW, the average retention time was 45minutes, and the stirring power index was 9.5 W·hr/kg.

Using a similar transverse type continuous stirring apparatus, it wascooled forcibly to 60° C. (cooling step).

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm, and the substance on 850 μm(substance not passed through 850 μm) was again crushed and mixed withthe substance under 850 μm to give a water absorbent resin powder (1),as a particle size-regulated product which was the total amount of thesubstance under 850 μm.

The obtained water absorbent resin powder (1) had 30.5 (g/g) of CRC,29.8 (×10⁻⁷·cm³·s·g⁻¹) of SFC, and 25.2 (g/g) of AAP. The standarddeviation of each physical property value was CRC: 0.16, SFC: 0.48, andAAP: 0.13. Additionally, these physical property values were averagevalues of the measurement carried out by sampling (5 points) every anhour until 5 hours were passed from the starting of the operation. Thephysical property values were determined in the same manner for thefollowing Examples and Comparative Examples. The results are shown inTable 1.

Comparative Example 1

The same operation was carried out as in Example 1 to obtain a waterabsorbent resin powder (2), except that the rotary valve installed inthe inlet and outlet sides of the transverse type continuous stirringapparatus (1) was uninstalled in Example 1. In other words, the periodicshielding was not carried out between the respective steps, wetting andmixing step-heating treatment step-cooling step, and completelycontinuous feeding was carried out in such a state. The water absorbentresin powder (2) obtained in Comparative Example 1 had 30.3 (g/g) ofCRC, 27.9 (×10⁻⁷·cm³·s·g⁻¹) of SFC, and 24.3 (g/g) of AAP. The standarddeviation of each physical property value was CRC: 0.45, SFC: 1.35, andAAP: 0.48. Table 1 shows the analysis results.

Example 2

The same operation was carried out as in Example 1 to obtain a mixture(2), a wet powder, except that 0.01 parts by mass of an aqueous solutioncontaining 10 mass % of polyoxyethylene (number of methoxy group: 20)sorbitan monostearate, a surfactant, was added to thesurface-crosslinking agent solution (1) in Example 1. That is, themixture (2) contained further 0.001 parts by mass of the surfactant inthe mixture (1).

The obtained mixture (2) was then surface-treated by a transverse typecontinuous stirring apparatus (2) having a downward inclined angle of2°, an aspect ratio of 2.4, a paddle rotation speed of 10 rpm, tworotary shafts, and stirring disks having scraping blades, and a surfaceroughness (Rz) of the inner surface of 500 nm (heat treatment step). Atthat time, the inside of the apparatus (2) was suctioned by a suctioninggas discharge apparatus having a bag filter, and the inside pressure ofthe apparatus was reduced to 1 kPa. A rotary valve (periodicallyshielding apparatus) was installed in the inlet (connecting part withthe mixing apparatus) and outlet (connecting part with the coolingapparatus) of the apparatus (1). According to a previous test, theposition of a discharge bank which gave an average retention time ofabout 45 minutes and an average filling ratio of 75% was measured, andthe discharge bank was set at the position as measured. A heating sourceused for the surface treatment was pressurized steam at 2.5 MPa, and thetemperature of the mixture (2) in the apparatus was measured by athermometer installed near the discharge part of the transverse typecontinuous stirring apparatus (2), and the steam flow rate wascontrolled to carry out the heating in such a manner that thetemperature was adjusted to 193° C. The total surface area of thestirring disks and the stirring shafts was 55.7 m² and the mass surfacearea ratio calculated from the total surface area (heat transfer surfacearea) and the treatment amount was 26.9 kg/m²/hr. The stirring power atthe time of the surface treatment was 32.0 kW, the stirring power inidling was 24.4 kW, and the stirring power index was 5.1 W·hr/kg. Usinga similar transverse type continuous stirring apparatus, it was cooledforcibly to 60° C. (cooling step).

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm, and the substance on 850 μm(substance not passed through 850 μm) was again crushed and mixed withthe substance under 850 μm to give a water absorbent resin powder (3),as a particle size-regulated product which was the total amount of thesubstance under 850 μm.

The obtained water absorbent resin powder (3) had 30.3 (g/g) of CRC,30.2 (×10⁻⁷·cm³·s·g⁻¹) of SFC, and 25.1 (g/g) of AAP. The standarddeviation of each physical property value was CRC: 0.13, SFC: 0.47, andAAP: 0.11. Table 1 shows the analysis results.

Comparative Example 2

The same operation was carried out as in Example 2 to obtain a waterabsorbent resin powder (4), except that the rotary valve installed inthe inlet and outlet sides of the transverse type continuous stirringapparatus (2) was uninstalled in Example 2. In other words, the periodicshielding was not carried out between the respective steps, wetting andmixing step-heating treatment step-cooling step, and completelycontinuous feeding was carried out in such a state. The water absorbentresin powder (4) obtained in Comparative Example 2 had 30.3 (g/g) ofCRC, 28.2 (×10⁻⁷·cm³·s·g⁻¹) of SFC, and 24.4 (g/g) of AAP. The standarddeviation of each physical property value was CRC: 0.43, SFC: 1.65, andAAP: 0.38. Table 1 shows the analysis results.

Example 3

Water absorbent resin particles (B) were produced by changing thethroughput to be 3 t/hr from 1.5 t/hr in Production Example 1, further,the same operation was carried out as in Example 1 to obtain a mixture(3), a wet powder, except that the surface treatment step (wetting andmixing step, heating treatment step, and cooling step) in Example 1 wascarried out in 2 lines (two respective apparatuses using in therespective steps were arranged in parallel, and throughput was 1.5t/hr×2) for 1 line of the polymerization step (throughput: 3 t/hr), andthen a water absorbent resin powder (5) as a product was obtained. Thewater absorbent resin powder (5) obtained had 30.5 (g/g) of CRC, 29.5(×10⁻⁷·cm³·s·g⁻¹) of SFC, and 25.2 (g/g) of AAP. The standard deviationof each physical property value was CRC: 0.28, SFC: 0.53, and AAP: 0.21.Table 1 shows the analysis results.

Comparative Example 3

The same operation was carried out as in Example 3 to obtain a waterabsorbent resin powder (6), except that the rotary valve installed inthe inlet and outlet sides of the transverse type continuous stirringapparatus (1) in 2 lines was uninstalled in Example 3. The waterabsorbent resin powder (6) obtained in Comparative Example 3 had 30.3(g/g) of CRC, 27.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, and 24.1 (g/g) of AAP. Thestandard deviation of each physical property value was CRC: 0.57, SFC:1.71, and AAP: 0.62. Table 1 shows the analysis results.

Example 4

The same operation was carried out as in Example 3, except that thesurface-crosslinking step (each one apparatus as a wetting mixingapparatus, a heating apparatus, and a cooling apparatus) was carried outin 1 line for 1 line of the polymerization step (3 t/hr).

The water absorbent resin particles (B) were continuously supplied at 3t/hr to a high speed continuous mixing apparatus (Turbulizer/1200 rpm)and at the same time a surface treatment agent solution (1) was mixed byspraying (wetting and mixing step). The surface treatment agent solution(1) was a mixed solution containing 1,4-butanediol, propylene glycol,and pure water. The surface treatment agent solution (1) was mixed withthe water absorbent resin particles (B) at a ratio of 0.3 parts by massof 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 2.7 partsby mass of pure water relative to 100 parts by mass of the waterabsorbent resin particles (B) to give a mixture (4), a wet powder.

The obtained mixture (4) was then surface-treated by a transverse typecontinuous stirring apparatus (4) having a downward inclined angle of2°, an aspect ratio of 2.5, a paddle rotation speed of 10 rpm, tworotary shafts, and stirring disks having scraping blades, and a surfaceroughness (Rz) of the inner surface of 500 nm (heat treatment step). Atthat time, the inside of the apparatus (4) was suctioned by a suctioninggas discharge apparatus having a bag filter, and the inside pressure ofthe apparatus was reduced to 1 kPa. A rotary valve (periodical shieldingapparatus) was installed in the inlet (connecting part with the mixingapparatus) and outlet (connecting part with the cooling apparatus) ofthe apparatus (4). According to a previous test, the position of adischarge bank which gave an average retention time of about 45 minutesand an average filling ratio of 75% was measured, and the discharge bankwas set at the position as measured. A heating source used for thesurface treatment was pressurized steam at 2.5 MPa, and the temperatureof the mixture (4) in the apparatus was measured by a thermometerinstalled near the discharge part of the transverse type continuousstirring apparatus (4), and the steam flow rate was controlled to carryout the heating in such a manner that the temperature was adjusted to200° C. The total surface area of the stirring disks and the stirringshafts was 46.5 m² and the mass surface area ratio calculated from thetotal surface area (heat transfer surface area) and the treatment amountwas 64.5 kg/m²/hr. The stirring power at the time of the surfacetreatment was 57.1 kW, the stirring power in idling was 24.3 kW, theaverage retention time was 45 minutes, and the stirring power index was10.9 W·hr/kg. Using a transverse type continuous stirring apparatushaving a similar shape, the water absorbent resin was forcibly cooled to60° C. (cooling step).

The water absorbent resin was classified by a sieving apparatus toseparate the substance under 850 μm, and the substance on 850 μm(substance not passed through 850 μm) was again crushed and mixed withthe substance under 850 μm to give a water absorbent resin powder (7),as a particle size-regulated product which was the total amount of thesubstance under 850 μm. The obtained water absorbent resin powder (7)had 30.1 (g/g) of CRC, 28.5 (×10⁻⁷·cm³·s·g⁻¹) of SFC, and 24.8 (g/g) ofAAP. Table 1 shows the analysis results.

TABLE 1 Comparative Comparative Comparative Example 1 Example 1 Example2 Example 2 Example 3 Example 3 Example 4 Water absorbnet resin (A) (A)(A) (A) (B) (B) (B) particle Feed amount [t/hr] 1.5 1.5 1.5 1.5 1.5 × 2lines 1.5 × 2 lines 3 Surface treatment agent (1) (1) (1) (1) (1) (1)(1) solution 1,4-BD [Parts by mass] 0.3 0.3 0.3 0.3 0.3 0.3 0.3 PG[Parts by mass] 0.5 0.5 0.5 0.5 0.5 0.5 0.5 W [Parts by mass] 2.7 2.72.7 2.7 2.7 2.7 2.7 Surfactant [Parts by mass] none none 0.001 0.001none none none Mixture (Wet powder) (1) (1) (2) (2) (3) (3) (4)Transvers type continuous (1) (1) (2) (2) (3) (3) (4) stirring apparatus(1) × 2 lines (1) × 2 lines Inclined angel [°] 1 1 2 2 1 1 2 Aspectratio 2.2 2.2 2.4 2.4 2.2 2.2 2.5 Paddle rotation speed [rpm] 13 13 1010 13 13 10 Surface roughness (Rz) [nm] 500 500 500 500 500 500 500Total surface area [m²] 24.4 24.4 55.7 55.7 24.4 24.4 46.5 Mass surfacearea ratio [kg/m²/hr] 61.5 61.5 26.9 26.9 61.5 61.5 64.5 Averageretention time [minute] 45 45 90 90 45 45 45 Average filling ratio [%]75 75 50 50 75 75 75 Temperature of mixture [° C.] 198 198 193 193 198198 200 near the discarge part Stirring power index [W · hr/kg] 9.5 9.55.1 5.1 9.5 9.5 10.9 Periodical shielding Rotary valve none Rotary valvenone Rotary valve none Rotary valve Water absorbnet (1) (2) (3) (4) (5)(6) (7) resin powder CRC [g/g] 30.5 30.3 30.3 30.3 30.5 30.3 30.1 AAP[g/g] 25.2 24.3 25.1 24.4 25.2 24.1 24.8 SFC [×10⁻⁷ · cm³ · 29.8 27.930.2 28.2 29.5 27.5 28.5 s · g⁻¹] Standard deviation CRC 0.16 0.45 0.130.43 0.28 0.57 AAP 0.13 0.48 0.11 0.38 0.21 0.62 SFC 0.48 1.35 0.47 1.650.53 1.71 Note) 1,4-BD: 1,4-butanediol/PG: propylene glycol/W: purewater Surfactant: Polyoxyethylene (20) sorbitan monostearate (added inform of an aqueous 10 mass % solution) [Parts by mass]: Amount per 100parts by mass of water absorbent resin particles Water absorbent resinpowder: Average value for 5 hours (sampling was done every one hour).

Example 5

The water absorbent resin powder (7) obtained in Example 4 waspneumatically transported by leading compressed air (dew point −15° C.and temperature 35° C.) into a pipe with a surface roughness (Rz) in theinner face of 200 nm, and wrapped. The SFC after the pneumatictransportation was 28.0 and the SFC decrease ratio was 1.8%.

Example 6

The same pneumatic transportation as in Example 5 was carried out,except that compressed air with a dew point of 20° C. was used. The SFCafter the pneumatic transportation was 27.2 and the SFC decrease ratiowas 4.6%.

CONCLUSION

Table 1 shows physical property deflection (standard deviation) incontinuous operation for 5 hours and physical properties (averagevalue). Table 1 shows the results (average value and standard deviation)of analysis of the products for every one hour.

As shown in Table 1, it could be understood that the physical propertiesof a water absorbent resin were improved and stabilized (reduction instandard deviation) by periodically shielding between a mixing apparatusand a heating apparatus, in other words, semi-continuouslysurface-crosslinking in the case where continuous surface-crosslinkingin which the continuous mixing apparatus for a surface treatment agentand a continuous heating apparatus were connected was carried out.

It was also understood that the physical properties of CRC, AAP, and SFCwere improved by carrying out the surface-crosslinking in 2 lines (2apparatuses arranged in parallel) for 1 line for polymerization as shownby comparison of Example 3 (1.5t/hr×2 apparatuses) and Example 4 (3t/hr×1 apparatuses).

It was also understood that pneumatic transportation using compressedair with a specified dew point was preferable from the results ofExample 5 and Example 6.

Example 7

The shielding intervals were changed as shown in Table 2 by changing therotation speed of the rotary valve in Example 1. The results are shownin Table 2. It could be understood from the results of the standarddeviation that the rotation speed of the rotary valve is preferably 10to 100 rpm and more preferably 20 to 100 rpm. The shielding interval ispreferably 0.01 to 0.1 minutes and particularly preferably 0.01 to 0.05minutes.

TABLE 2 Rotation speed (rpm) 10 25 50 100 Shielding ratio (%) 27.5 11.05.5 2.8 Shielding interval (min.) 0.10 0.04 0.02 0.01 CRC 30.5 30.5 30.330.4 SHC 29 29.8 29.7 29.5 AAP 24.9 25.2 25.1 25 Standard deviation σCRC 0.21 0.16 0.13 0.13 SHC 0.65 0.48 0.45 0.44 AAP 0.21 0.13 0.11 0.12

Comparison with Conventional Techniques

The physical properties of a water absorbent resin were improved andstabilized (reduction in standard deviation) by carrying out periodicshielding and controlling the stirring power index in this application,as compared with those in Patent Documents 1 to 41, conventional surfacetreatment techniques. For improvements in the apparatus, for example,techniques of using specified mixing apparatuses for mixing apparatusesfor surface-crosslinking agents (Patent Documents 26 to 29) are knownand also techniques of heating apparatuses for carrying out reaction ofwater absorbent resins and surface-crosslinking agents (Patent Documents30 and 31) are known; however, such improvement techniques in theapparatus do not indicate the present application and correspond tocomparative examples in the present invention.

INDUSTRIAL APPLICABILITY

A water absorbent resin with high physical properties, particularly,high liquid permeability is provided by continuous production in a hugescale (e.g., 1 t/hr or more).

EXPLANATION OF REFERNCE NUMERALS

10: Driving apparatus

20: Transverse type drum

30: Raw material supply port

40: Heat medium inlet

40′: Heat medium inlet

45′: Heat medium outlet

50: Water absorbent resin discharge port

70: Rotary shaft

80: Stirring disk

80 a: Stirring disk

80 b: Stirring disk

81: Carrier gas introduction port

85: Gas discharge port

90: Scraping blade

90 a: Scraping blade

90 b: Scraping blade

100: Stirring apparatus (stirring means)

1. A method for producing a polyacrylic acid (salt)-type water absorbentresin, comprising a step of preparing an aqueous monomer solution of anacrylic acid (salt), a step of continuously polymerizing the aqueousmonomer solution, a step of finely shredding a hydrous gel-likecrosslinked polymer during or after polymerization, a step of drying theobtained particulate hydrous gel-like crosslinked polymer, and a surfacetreatment step of adding and reacting a surface treatment agent to andwith the dried water absorbent resin powder, wherein in the surfacetreatment step, a continuous mixing apparatus for the surface treatmentagent and a continuous heating apparatus are connected and periodicalshielding is carried out between the mixing apparatus and the heatingapparatus.
 2. The production method according to claim 1, wherein theperiodic shielding is carried out at a shielding interval of 0.001 to 5minutes and a shielding ratio of 1 to 80%.
 3. The production methodaccording to claim 1, wherein in the surface treatment step, thecontinuous heating apparatus and a continuous cooling apparatus areconnected and periodical shielding is carried out between the heatingapparatus and the cooling apparatus.
 4. The production method accordingto claim 1, wherein the periodic shielding is carried out by using atleast one selected from a gate, a valve, a damper, a rotary feeder, anda table feeder.
 5. The production method according to claim 1, whereinheating treatment in the surface treatment step is carried out in atransverse type continuous stirring apparatus having stirring meansincluding a feeding inlet and a discharging outlet of a water absorbentresin and one or more rotary shafts having a plurality of stirringdiscs, and heating means.
 6. The production method according to claim 1,wherein the surface treatment step has two or more lines for one line inthe polymerization step.
 7. The production method according to claim 1,wherein the production amount of the polyacrylic acid (salt)-type waterabsorbent resin is 1 t/hr or more.
 8. The production method according toclaim 1, wherein the surface-crosslinking agent is a dehydrationreactive surface-crosslinking agent and the heating treatmenttemperature is 150 to 250° C.
 9. The production method according toclaim 1, wherein the surface treatment step is carried out in reducedpressure.
 10. The production method according to claim 1, wherein thewater absorbent resin has 20 g/g or higher of water absorption againstpressure (AAP) for an aqueous 0.9 mass % sodium chloride solution at apressure of 4.8 kPa, 1(×10⁻⁷·cm³·s·g⁻¹) or higher of flow conductivityof 0.69 mass % physiological saline solution (SFC), and 20 g/g or higherof water absorption under no pressure (CRC).
 11. The production methodaccording to claim 1, wherein the periodic shielding is further carriedout also in the outlet side of the heating apparatus.